Richard wrote: "should we not first ask 'What is Blues'?"

The black folk songs that had the word "blues" in them as of about 1909, the existence of which caused anyone to start talking about "blues songs," in about 1909, were apparently generally first-person sad songs with repetitive lyrics within the stanza, and chord progressions similar to I-I-I-I-IV-IV-I-I-IV-IV-I-I-V-V-I-I or I-I-I-I-IV-IV-I-I-V-V-I-I, rather than the quite different chord progressions found in countless black folk songs of the era. Songs with those characteristics that were reported to predate 1909 include "Got No More Home Than A Dog," the "Poor Boy Long Ways From Home" family, and the "K.C. Moan" family.

For anyone who wants to define "blues music" broadly, to include the likes of "John Henry"... we don't have evidence that that broadly defined "blues music" started in the Yazoo-Mississippi Delta either.