Carl Sandburg had an important interest in blues, and he and the Lomaxes were friendly (e.g. Sandburg and J. Lomax were together when they heard Martinez sing his blues in Austin, included in the Songbag), and Alan may have been influenced by Sandburg on this stuff, because Sandburg had a "Mississippi Blues" he liked to perform himself and talk about as early. But of course Sandburg didn't promote the Mississippi myth the massively influential way Alan did, we know that.