I just find it interesting that there are population segments/ethnic groups that have not changed their speaking style over a few hundred years and assimilated with the majority population, while there are others who have.
Bear in mind that in the southeast around the year 1900 where virtually all African-Americans lived, no one, black or white, was speaking in terms you've identified as "mainstream" English. You would start the clock on assimilation into northern English to the time of the Great Migration and then the spread of radio and television, and even then, our media has a diversity of voices and patterns that people are exposed to.
I also wouldn't presume that rural Southern speech of 1900 was the same as that of 1750, but I don't really know.
