"they were playing one strand of the popular music of the day" Someone who sang "K.C. Moan" or "Poor Boy Long Ways From Home" in 1905 was singing a folk song, not a pop song.

"Who were the pop musicians of the day for the african americans of the day, if not them?" People like Bert Williams. We could talk about "stage musicians" if that gets more to the point for some: who can give a plausible example of a stage musician who had helped invent blues music as of 1905, when blues music already existed among folk musicians, such as the railroad workers Elbert Bowman heard? No one can. Elijah can't, Abbott and Seroff can't, Calt couldn't, etc. Now if you take 1910 and Johnnie Woods, e.g., he _is_ an example of a stage musician who was singing blues as of 1910. And 1910 isn't 1905.