Great discussion (the good faith posts, I mean).
A few comments and a few questions (not a native speaker but I'll try):

Like Joseph, I am quite convinced Alan Lomax initiated this claim, and at least do not know of anyone making it before him, let alone anyone this influential.

Also like Joseph, it would seem natural to me that the field recordings A. Lomax had made in the Delta would have played some role in his focusing on this specific area.

By the time A. Lomax first made the "Delta claim" (which, to my knowledge and to Joseph's it seems, would be 1947), he had been to the Delta at least three times (his father, five times): once with his father in August 1933, and twice with others, in the summers of 1941 and 1942. On these last two trips, of course, he had recorded Son House, Muddy Waters and illustrious others. Needless to say, though, that by 1947, none of them was even close to being a "blues legend."

One could also go back to A. Lomax’s epiphany, when John Hammond “put [him] on to Robert Johnson,” – “Johnson’s recordings stood out as the finest examples of the blues along with those of the great Blind Lemon Jefferson in the twenties” – at some point between June 1938 and March 1939, at which time A. Lomax was planning to make an album on Johnson with Hammond, and was urging his father to “investigate one Robert Johnson of Robinsville [sic], Mississippi.”

Why “investigate” Robert Johnson as of March 1939, and why A. Lomax would repeatedly claim he went to the Delta in the early 1940’s “to look for Robert Johnson,” when Hammond had announced Johnson’s death (though dating it inaccurately) by December 13, 1938, is unclear to me. Be that as it may, that Hammond replaced Johnson with Big Bill Broonzy for his historic and historical “From Spirituals to Swing” concert, attended by A. Lomax ten days later, may not have been without consequences on the latter’s then forming blues mythology.

Among other factors that may have contributed to A. Lomax’s “Delta birth claim,” one could think of his self-admitted romantic nature, his general aspirations and political views.

But regarding a more immediate trigger, I find it hard to think that Big Bill Broonzy, and more specifically the conversation that A. Lomax recorded on March 2, 1947 in NYC of Big Bill Broonzy, Memphis Slim and Sonny Boy Williamson (I), could have had nothing to do with A. L’s taking the leap to make the "Delta claim" a few months later, nor with providing support to said claim in the following years. (I now see that Vic Hobson, who often makes good connections, mentions this recording, but he strongly underestimates its impact on A. Lomax, imo, as well as Broonzy’s specific influence. Also, and it might be related, Hobson is factually mistaken about the Common Ground article)

Chronology is not causality, yet, the following seems quite striking to me:

March 1946: An article by Big Bill Broonzy, "Baby, I Done Got Wise," is published in The Jazz Record, in which Broonzy starts to claim that he used to know a "blues singer"/ fiddler in Arkansas, in the 1910’s, known as “See See Rider” because he used to sing the eponym song.

July 1946: Big Bill Broonzy appears for the first time in a People's Songs hootenanny, in NYC.

October 1946: BBB gives Alan Lomax a nine-page handwritten biography in which he again mentions the alleged Arkansas fiddler called “See See Rider.”

November 9, 1946: BBB is for the first time on the bill of a concert organized by A. Lomax (with People's Songs), at Town Hall, NYC - Great success, for the event and for BBB.

March 1, 1947: BBB, Memphis Slim and Sonny Boy Williamson are on the bill of another A. Lomax / People's Songs event in NYC. The same evening, they have dinner and sleep at A. Lomax's place in Greenwich Village.

March 2, 1947: The next day, A. Lomax brings the three musicians to Decca Studios, and records them in what is partly a collective interview, partly a conversation, interspersed with performances. Among other subjects, they evoke the origins of the blues, which they do not say was specifically born in the Delta, though, but more generally "not (…) in the North," "not (…) in the East," but "in the South" and at one point "Down South." On or near this date, Lomax also records BBB alone, for an oral biography.

December 23, 1947:
Folk Song USA, edited by Alan Lomax, is copyrighted. It is specifically in notes to "See See Rider" - the song - that A. Lomax, for the first time, it seems, makes the claim that the blues was born in the Delta. Just a few paragraphs before stating that “See See Rider” is “said to have been composed by an Arkansas Negro in the early part of this century” (cf BBB’s story), Lomax writes: “A folk blues has a simple tune (…). No Tin Pan Alley sharpie had anything to do with this blues, (…). It came out of the great valley of the blues, the Mississippi Delta, where the earth and the people are equally fertile and burdened with troubles.” A few lines later, he concludes a very brief attempt at a history of the blues with “Now this lonesome melody of the Mississippi has uncoiled in the ear of the whole world.”

Incidentally, he also mentions that “[W. C.] Handy, traveling through rural Arkansas, heard this music [the blues],” which, given that Handy almost never mentions Arkansas specifically, much less as a place where he would have first heard blues songs, and given that A. Lomax had likely read both Handy’s Blues: An Anthology (1926) and Handy’s autobiography (1941) by 1947, on top of making a recorded interview of him for the Library of Congress in 1938, seems indicative to me, but some may find it far-fetched, that A. Lomax had clearly decided at this point to prefer Broonzy’s claims over Handy’s, against all common sense. While not stated clearly, it also seems to indicate that Lomax’s conception of the Delta was not so rigorous here as to exclude Arkansas, or at least eastern Arkansas. Could the fact that he conflates the Delta with a “valley” also imply that by 1947, his conception of the “Delta” was even broader than that? Implying that pretty much any place at least two of his new informants, namely BBB and Memphis Slim, would mention, whether in Mississippi or Arkansas, or even maybe West Tennessee could then conveniently be included into this fascinating “Delta” concept?

January and February 1948*: On January 23, A. Lomax and his father meet in the Delta for a recording / common conferences trip. A few hours later, John A. Lomax has a heart attack and dies three days later in Greenville, MS, on the day recordings had been planned at Parchman Farm, the Mississippi State penitentiary. Father and son had made recordings there together, more than 14 years before, during Alan’s first field trip, and John had returned three times in the late 1930’s. After John’s funeral in Austin, TX, Alan comes back to the Delta and makes the planned recordings at Parchman, on February 9, 1948. The idea of this 1948 trip had formed at least by January 1947, likely in John’s mind more than Alan’s, and therefore cannot be – or not totally be – imputed to Parchman and chain gangs being mentioned in the March 2, 1947 conversation. It is likely, however, that Alan had said conversation in mind when making these recordings, which he would use almost ten years later, both as illustrations to the same conversation in the Blues in the Mississippi Night LP, and in a dedicated LP: Negro Prison Songs. (*I have yet to see strong evidence that any Lomax went to the Delta at any time in 1947, let alone in late 1947, as is often claimed. More than unlikely imo. Glad to share evidence if asked)

March 1948: Folk Song USA is published.

Summer 1948:
An article by A. Lomax, “I got the blues,” is published in Common Ground. It is structured around the March 2, 1947 conversation, including many extracts verbatim, but set in East Arkansas, close to Memphis, and not NYC, with a fully invented story about Lomax and the three musicians first meeting in and then being chased from Beale Street (Memphis) that same evening, and with the musicians’ names changed, in order, Lomax would explain, to protect them and their families.

In this article, A. Lomax, once again (contrary to what Hobson strangely claims), in fact three times, makes the claim that the blues was born in the Delta / Mississippi. He first writes: “Child of this fertile Delta land, voice of the voiceless black masses, the blues crept into the back windows of America maybe forty years ago and since then has colored the whole of American popular music.” He then adds, using almost the exact same words as in Folk Song USA: “Now the blues is a big, lonesome wind blowing around the world. Now the whole world can feel, uncoiling in its ear, this somber music of the Mississippi.” The last one: “Here was Natchez [Big Bill Broonzy], who had helped to birth the blues forty years ago in this same Delta country” – that is, as late as 1907 / 1908, at age 3 to 5, when he himself would repeatedly claim the blues existed before he was born (true or invented birthdate).

Finally, regarding BBB and Memphis Slim’s influence on A. Lomax’s claims: after stating that the blues was an evolution from work songs, A. L. writes, “Here, from the experience of Leroy [Memphis Slim] and Natchez [BBB], had come confirmation for my own notions about the origins of the blues.” If Broonzy and Chatman’s “musicological” insights regarding the birth of the blues - however young they were, and no matter that they did not even claim that their stories actually informed the birth of the blues - were so relevant to Lomax, couldn’t it have been that their geographical accounts were also likely to influence him? Not that any of them, ironically, had been born nor raised in the Delta (strict definition) by the way. But their mentioning names like Parchman, or Merigold, or Scott, MS, or the “Loran [Lowrance] brothers’” levee camps “all around Memphis,” or even Arkansas surely must have had some kind of impact – the latter since, here again, A. L. seems to consider at least East Arkansas as part of the Delta (e.g., “Now we sat together in the Delta night”).

1950: Still on A. Lomax’s geographical conception of the Delta, I am not exactly sure what to make of the following in Mister Jelly Roll: A. L. describes New Orleans as “where the Mississippi Delta washes its muddy foot in the blue Gulf.” Isn’t something’s foot part of said thing? Is he here referring to the “Mississippi River Delta,” in which case the “foot” image does not make much sense? Or is there such a thing as a “leg” let’s say, in A. L.’s mind, corresponding to a very broad conception of the Delta, something like a “Natchez – Little Rock – Memphis” triangle, and then South Louisiana that would be a “foot”?

September 21, 1951: While on his first European tour, BBB records “Blues in 1890” in Paris, France, for Vogue. This is the first of several recordings he would make of a half-spoken half-sung version of “Joe Turner,” which, for instance, Abbe Niles says was “quite likely” “the grandfather of (…) all [blues]” in W. C. Handy’s Blues: An Anthology (1926). Handy, who claimed to have heard it in his youth, had performed it for A. Lomax, giving a clear and credible account as to its Tennessean origins during their 1938 recorded interview, consistently with the countless other times he mentioned the song throughout his life. BBB, on the other hand, would connect it to Mississippi in an interview with A. Lomax a few months later, and still later claim it was the first blues he had ever heard.

September 22, 1951: The next day, BBB gives his first two concerts ever in the UK (on the same day), at Kingsway Hall, London. Alan Lomax, then residing in Scotland, comes to London to introduce the evening show, later described as “largely a conversation between old friends, Alan Lomax and Big Bill, with songs from both.” While the attendance is not strong, the press is ecstatic.

September 29, 1951: In a Melody Maker interview, BBB makes the connection between the blues and Mississippi, though not as clearly as A. Lomax would: as for learning the blues, BBB says, “you got to be born a Negro in Mississippi and you got to grow up poor and on the land.” At this point, the only American “guitar / non-jazz” bluesmen to ever have performed in the UK, it seems, were Lead Belly and Josh White, so that BBB’s words seem to have made quite an impression and been taken as valuable testimony. Regarding Arkansas’ association with Mississippi into a great “whole,” again, Broonzy does not claim that Arkansas belongs to the Delta but says “The blues, they’re field hollers way down in Mississippi and Arkansas,” which happens to also be perfectly in line with Lomax’s insistence on the role of hollers and work-songs in the emergence of the blues.

Also, mentioning that early blues songs were not initially called blues, Broonzy adds “That’s one thing Handy’s tellin’ the truth about,” which – besides being a good laugh – is the confirmation of two things imo: that Broonzy was fully aware of Handy’s claims at this point, possibly helped in this by A. Lomax, and that there was a clear, if not wholly explicit, opposition, by then, between two sides of (alleged) “blues witnesses” – the supposedly “authentic” “Big Bill Broonzy and A. Lomax” side, all about rough field work and hollers, and the supposedly “middle-class / Broadway / Tin Pan Alley” side, according to the caricature that was being made of Handy. Handy and Broonzy’s antagonist accounts on the genesis of “Joe Turner,” which Broonzy briefly mentions again in this interview, is a good indicator of this opposition imo – and of who is to be taken seriously.

November 28, 1951: A. Lomax broadcasts parts of the March 2, 1947 conversation in the third episode, “Blues in the Mississippi Night,” of his BBC program, “The Art of the Negro.” According to the manuscripts, in the intro, Lomax says: “The blues is a Mississippi of song that pours out of the central dark valley in America and uncoils in its sadness in the ear of the whole world” - in which we find the same image as in Folk Song USA and the Common Ground article, as well as the fact that Mississippi “Delta” and Mississippi “valley,” are alternatively, if not interchangeably, used by Lomax as the region where the blues was born. He also mentions and broadcasts in the same program “songs of the penitentiary” in “Mississippi,” in other words, Parchman, in connection with the three musicians’ accounts on the “background of the ‘blues.’”

May 13, 1952: Back in Europe, BBB is interviewed by A. Lomax, in Paris, France. He performs “Joe Turner,” after which they evoke the song, which BBB connects to his legendary uncle Jerry (the one still living at age 103 by 1952 but who had been killed by 1947). In this account – one of many contradictory and sometimes self-contradictory accounts by BBB on the song’s genesis, some of which are offered as evidence in many influential blues books, regardless – Big Bill’s mother, it seems, told him that Uncle Jerry used to play the song in 1892, in Mississippi, we infer, since according to BBB, his family was living there at the time. BBB would later “confirm” that he first knew it to have been sung in Mississippi.

In or just before July 1957: A. Lomax releases the LP Blues in the Mississippi Night in the UK, made of extracts of the March 2, 1947 conversation, intertwined with musical sequences, some by the three musicians, some by others, including prisoners recorded at Parchman in 1948. In the liner notes, evoking the blues, Lomax writes: “So in the Yazoo Delta Country, south of Memphis, where the conditions described on this record were typical, there emerged this new dance music, the work of many hands and voices.” Then he proceeds to explain how, according to him, work-songs and hollers evolved to finally become the blues, and adds: “But it must be remembered that all these blues were for dancing.” Of the record, he says: “This, then is the story of the blues told by men who have lived the blues and created the blues.” While none of them, obviously, “created the blues,” stating so regardless, in addition to overstating these men’s connection to the Delta – especially when defined as strictly as above, surprisingly – certainly did its share to spread the Delta birth theory.

October 1957: A. Lomax releases the LP Negro Prison Songs from the Mississippi State Penitentiary in the US and the UK (where it is titled Murderer’s Home), documenting the Parchman recordings of February 1948. In the liner notes, he writes: “Here is the dark, fertile soil which gave rise to the blues. Indeed, this recording, made in the heart of the Mississippi Delta where the blues took shape at the turn of the century, provides the background for America’s most important song-form.”

May 1959: Blues in the Mississippi Night is released in the US.

(Many BBB facts listed above can be found in Bob Riesman’s seriously sourced BBB biography: I Feel So Good: The Life and Times of Big Bill Broonzy. Will gladly give sources for other facts if asked, this post is already long).

Six months later, two books are published almost simultaneously (Nov. and Dec. 59, respectively): The Country Blues by Sam Charters and Jazz, a collection of essays, edited by Nat Hentoff and Albert McCarthy.

In the (obviously very influential) former, as mentioned by Joseph, Charters strongly reenforces the Delta birth claim, while in the latter, Paul Oliver, in his essay "Blues to drive the blues away" is far more cautious and while acknowledging that among states where the blues might have been born, “Mississippi is perhaps most frequently cited,” suggests polygenesis as a strong possibility – not a very reasonable theory either, imo, but at least not subjected to Lomax’s claim.

Which brings me to my questions:

How do you feel both mainstream and “scholar” adhesion to the “Delta claim” evolved over time?

I have a general feeling, but did not investigate this and might be totally wrong, that the claim’s popularity grew from 1947 on, to reach a climax between the late 50’s and the late 60’s, let’s say, even in supposedly “informed” circles, and that while Lomax and Charters never ceased promoting it and mainstream publications kept capitalizing on a selling concept, it still slowly declined ever since, as shown imo in Paul Oliver’s Story of the Blues (1969), Jeff Todd Titon’s Early Downhome Blues (1977), David Evans’ Big Road Blues (1982) etc.

Also, I would tend to judge Oliver a bit less severely than you do on the matter, Joseph. I hear you on “axiomatic” – Charley Patton was certainly axiomatic of Charley Patton – but among post-1947 influential authors, it seems to me that Oliver did much to (politely) reject the idea that there was evidence for the Delta claim.