Songs who lyrics mentioned having the quote "blues," in general, were occasionally being written during e.g. 1850-1901, because to have "the blues" meaning to be depressed was a well-known expression in the 19th century. Black folk songs that mentioned having the quote "blues" in the lyrics became popular enough by about 1909 that people began talking about those songs as so-called "blues songs" as such in about 1909 -- and on the evidence, the same was not true as of 1904.

But the "blues songs" about having the quote "blues" that were rapidly becoming better known during the 1909 to 1916 period were very, very similar to songs without the word "blues" in their lyrics that apparently went back to before about 1904, along the lines of "Poor Boy Long Ways From Home" and "Got No More Home Than A Dog." Even as of the 1910s, in folk music, complaining that you had the quote "blues" seems to have been no more popular, in what we generally think of as blues songs, than complaining you were quote "worried" seems to have been, in what we generally think of as blues songs.

John Jacob Niles wasn't a reliable source of information. For some reason Sam Charters took blues material that was clearly cited as collected in the 1910s and reimagined "1904" in his famous 1959 blues book, so that's another example of a red herring. So is "Nobody There"; it gets mentioned a lot because David Evans mentioned it in connection with blues in his well-known 1982 book, but that fragment doesn't have a known special connection to blues relative to a lot of other material of about the same time. (Also, Gates Thomas was honest, unlike Niles, but his guesses at dates, as he remembered music he'd heard, don't seem to have been particularly reliable. Similarly, Handy was honest, but in his 1941 book his story about being in St. Louis in "1892" because he had left Bessemer because of the Panic Of 1893 doesn't add up chronologically.)

The idea that "8-bar blues" as such is a "thing" seems to have largely arisen out of Leroy Carr's popularity (although of course there were certainly sad songs with 8-bar structures in e.g. the 1890s). His version of "How Long Daddy, How Long" was retitled "How Long - How Long Blues" and was a huge hit.

Regarding all the songs that were on the V chord as the first half of the progression ended -- you know, like generally similar to the song that goes "This morning, this evening, so soon" -- that was a cliche 19th-century thing, and overall is very negatively correlated with which tunes black musicians generally called "blues" tunes.

The belief of the researcher of black folk music Newman White (born in the South in 1892) that the two most popular lyric forms in blues as of the 1910s were AAB and AAAB seems to be accurate based on other evidence. Sixteen-bar music very similar to "One Dime Blues" by Lemon Jefferson was more popular before about 1925 than is generally recognized, and routinely had AAAB lyrics, but could have other lyric forms such as ABBC. Musicians who knew that 16-bar type of blues (which is almost never on V as the first half of the progression ends, is basically just like a 12-bar blues that's been expanded to have another IV-IV-I-I bit) include Leadbelly, Furry Lewis, Rev. Gary Davis, Sara Martin, Peg Leg Howell, Henry Thomas, Bo Carter, Skip James, Big Bill Broonzy, L.V. Thomas, Texas Alexander, the Mississippi Sheiks, Barbecue Bob, Pink Anderson, Blind Boy Fuller, Jesse Fuller, Tom Darby, Bobby Grant, Marshall Owens, Cow Cow Davenport, Clarence "Jelly" Johnson, Butch Cage, Mance Lipscomb, Lucille Bogan, Henry Whitter, Wiley Barner, Sonny Terry, the Memphis Jug Band, Louis Armstrong, Thomas Shaw, Bayless Rose, Lightnin' Hopkins, Cecil Barfield, Elizabeth Cotten, Andrew Baxter, Bill Moore, Euday Bowman, J. Neal Montgomery, Wilton Crawley, Clarence Williams, Charles H. Booker Jr., William Harris, Daddy Stovepipe, Charley Jordan, and many, many others.