This probably goes without saying, but you would think that a poem titled "Ma Rainey" would talk a lot about Ma Rainey. But here's the weird part: she's hardly even in the poem. We know she sings, and that's about it.
What we get of Ma is a good picture of who her fans are and how she inspires them. We share the experience of a Ma Rainey concert. When we go to a concert, the band's name is on the ticket, but sometimes we connect more with the other fans than with the actual band. Sometimes the band is even on a stage behind big, scary dudes and fences.
But in Ma Rainey's case, it's clear that these concert-goers feel a deep connection to the lady on stage (she's not stuck behind police barricades). Whether or not she reciprocates the connection is kind of beside the point. We're not talking about the real Ma Rainey here; we're talking about the a Rainey of imagination. What these fans see in her far more important than the woman herself.