The blues players were travelers. It's unprovable where it started. It was an extension
of field hollers and work songs. The twelve bar blues may have started in the Delta
or other places. A field holler can be structured as an AAB form, the first line
repeated than concluded on the third line. The blues scale was probably originally
based on an African mode or scale untempered. The flatted third, fifth and seventh notes of the Euro-based scale were not exactly tempered. The flatted seventh, if you listen closely is slightly flat of the flatted seventh European scale.

I think the 12 bar blues was probably standardized in the repertoire of New Orleans jazz
musicians.

The Delta undoubtedly generated a blues style as did the Piedmont style in N.C.

I think there was an interaction between the blues performers and the New Orleans
Jazz. There was ragtime before what was called jass materialized.

Field hollers were not restricted to any place in the South geographically.

Alan was influential in his take on the blues but he was not a musicologist.
He didn't have the theoretical musical chops.

The Delta may or may not have started the blues as it became recordings and sold.
It's significant that the African-American modal pattern emanating from West Africa
has found it's way into a variety of hybridized music, some of it mixing Euro and Afro
musical styles. I submit that two of the greatest blues musicians were Charlie Parker
and Wes Montgomery whose playing is rooted in the blues.

The question remains, in what form are we referring to when we speak of the blues?
There are hollers, moans, 12, 8 and 16 bar blues. The cabaret blues of Bessie Smith
is a variation of blues.

Mack McCormick, Alan, Sam Charters et. al. may be scholars and academics but you
might go down the rabbit hole as asking where did music begin? Maybe blues
started with the Neanderthal man on the mouth bow.

This question can not be answered. Delta blues is significant however.