I tend to think Abbe Niles did.

Early songs that we now consider blues had been recognized in print as folk songs since Odum in 1911.

Yet, I can’t find the phrase “folk blues” before February 1926, when it is seen in announcements for Blues: An Anthology (edited by W. C. Handy, with an introduction and notes by Niles), which was eventually released in May of the same year:

“The publication of W. C. Handy’s anthology, 'Blues,' is scheduled for some time in March or April . . . . The book . . . is an anthology of jazz music from the early negro folk blues to modern music . . .” (The Salt Lake Tribune, February 26, 1926, p. 11)

Since (to my knowledge) Handy had not previously used the phrase in about ten years of articles and interviews, whereas it is constantly used by Niles (himself quite new to blues commentary at this point) in his introduction and notes, it would seem likely that Niles coined it, rather than Handy.

Did I miss something?