When Ma Rainey (1)
The speaker assumes that we know who Ma Rainey is, just as you would assume someone knows The Beatles or Beyonce. He may also assume that you know she is an African-American performer in the Jim Crow South and most of her fans are going to be black, too.
Folks from anyplace (3)
The word folk has some very specific connotations. Usually, it gives the reader a sense of down-homeness, of simple dignity, and that's definitely how it's being used here. Brown might be doing that on purpose, because the word was often used in this sense, back in the early 20th century. For example, W.E.B. Dubois wrote a very important book called The Souls of Black Folk that championed Black culture as valuable to the culture at large.
Dey comes to hear Ma Rainey from de little river settlements,From blackbottom cornrows and from lumber camps; (19-20)
Here the speaker mentions a few traditionally black communities in the south. River settlements, lumber camps—they're all places where working-class African Americans could make a living and share a space.