Josepp's list of Scott Joplin and James Scott does omit another great black musician who, in another (less racist) culture could have made a greater mark on 'classical' music, and that is Thomas (Fats) Waller.
He wrote both serious organ music and an opera (OK, a musical).
He still had to make his living playing rent parties and clowning, at which he was supreme.
I have remarked before that an African former student of mine, on finding me listening to Robert Johnson, and learning that it was an American artist, was surprised and said he would have said that he would have though it was someone from Mali. Of course, there is no way we can be sure that Malian musicians do not have a large collection of early blues records and have been influenced that way, but the fact remains that early blues has a familiar sound and feel to a non-hyphenated African musician.
Cheers
Dave