I live in that band of minority voters in Alabama. I never have figured it out. Most of the them are good people. This is the area that had the really large cotton plantations before the War. I guess most blacks just stayed on the plantation, so to speak.
The “Black Belt” is the area where blacks live in high numbers in the American South.
You are indeed correct about Africans staying put after emancipation. There are other dynamics too, like WW2 when many moved to the San Francisco Bay Area to do menial labor.
But for the most part, black people live where their ancestors lived.
Another thing I find interesting is agriculture and black people overlap, and conversely places that historically had very little agriculture to this day possess very few black people. Arizona, New Mexico, Alaska, et al.
People move for a reason. I think the term for this field of study is “Cultural Geography”.