Can i recommend a book - Elijah Wald's Escape from the Delta.

Wald explains the subject better than i ever could.

Wald's proposition is that to understand the development of the blues, we need to examine the pressures and circumstances under which the music was produced, and these vary greatly. I would hazzard a guess, that Wald would say that the mixture history threw up in those years was too various to generalise over much.

each truth demands its own space.