I think Hootenanny is missing the point. The Patton and James tracks she mentions come from quite a lot later, and if you really want something interesting, try John Lee Hooker's take on "I Left My Heart in San Francisco". A lot of it can be traced back to John Hammond's presentation of Big Bill Broonzy at the "Spirituals to Swing" concert, when despite the fact that Broonzy had been resident in Chicago for years and normally wore flash suits, Hammond got him to dress in overalls and be anounced as coming straight from his farm in Arkansas! The Bluesman as noble savage!

As I said, read Wald's book.