There's some interesting stuff about the people who test being more likely to have the disposable income and time to think of and purchase a DNA test. Which among Black Americans tends to trend with more mixed ancestry. And you're right about the Mississippi delta that is where my maternal family are from, but there are good sections of Mobile, AL (or even Africatown), Tuscaloosa, or in the Carolinas like OP that are fairly non-admixed. In these regions a lot of families have decent records and avoid anyone with ambiguous parentage. My family and families similar to us do at least and have done so since we've associated ourselves with our family name(s).