Some more examples, in addition to what's quoted below:

The country musician Elbert Bowman rarely encountered black people in his small Tennessee town when he was young, but remembered that during 1903-1905 a railroad was built through the area, and black laborers building it sang a member of the "K.C. Moan" family.

John Lowry Goree included a variant of "K.C. Moan" among the folk material he said he encountered in Alabama before he moved to Texas in 1903.

"Howard Odum's collecting of black folk songs during 1905-1908 included blues songs about having the 'blues,' such as the 12-bar 'Knife-Song.' (Wald has acknowledged to me that Odum collected what he would call blues songs during that period.) E.C. Perrow's articles collecting folk lyrics, independently of Odum's research, included blues lyrics about having the 'blues' sung in 1909, notably similar to lyrics collected by Odum. A guitarist on a levee performed a number called 'I Got The Blues' in 12-bar form within Antonio Maggio's earshot in 1907. We know Emmet Kennedy knew a variant of 'Poor Boy Long Ways From Home' during the second half of the first decade of the century ('I feel certain that [that song] goes back further than 1905,' he claimed), and he recalled that he first heard it done by blacks on the street; Gus Cannon recalled knowing that song around then too. Famously, W.C. Handy recalled hearing what we'd generally call a blues performance from a musician in Tutwiler, in roughly 1904 (his memoir doesn't say it was '1903'). Mance Lipscomb recalled that he knew 'All Out And Down' in about 1909. Elizabeth Cotten learned 'Going Down The Road Feeling Bad' in roughly 1909; Handy's writings show that he believed that song was around before that. The 12-bar 'Got No More Home Than A Dog' that Handy claimed he encountered before 1900 is similar to roustabout music apparently from around the same time that was collected by Mary Wheeler, and is also similar to other music that the likes of Leadbelly apparently learned before 1910. Wheeler and Texas Alexander (in 'Blues') independently both had the 'Merry Widow hat' stanza, and those hats were a brief fad in about 1908.

Southerners who were old enough to know such as W.C. Handy and Perry Bradford claimed that blues music had originated among folk musicians. '[B]lues originated from old... folk lore songs.' -- Bradford, 1921. '"Blues" music... is of negro origin.... [I]t is from the levee camps, the mines, the plantations, and other places where the negro laborer works that these snatches of melody originate.' -- Handy, 1919.

Some songs we'd call blues songs have demonstrable roots in pre-1905 folk music; e.g., the Memphis Jug Band's 'A Black Woman Is Like A Black Snake' is based on the folk song 'Hop Joint,' which John Hurt recalled well he knew before 1905."