As a "poor Southern white", I can assure you that while similar sounding and often mutually intelligible, Ebonics and the various Southern dialects are not the same.
Mrs BN is from New England. After our first year or so of married life in Chicagoland area (1971-72), we moved to Middle Georgia (like Middle Earth, but more of us wear shoes than do Hobbits). One day while driving in the country, we stopped to ask directions of a young black boy. I got out of the car and talked a while with him. When I returned, my Yankee wife asked, "What language were you speaking?"
"English"
"Both of you?"
"Yes"
I did not understand a word either of you said!"
(We won't even get into Geechee!)
During the Jazz age, a lot of southern blacks moved to the northeast and the great Lakes states.
The southern drawl was a selling point particularly in the music world, so an attempt was made to preserve it across the generations. If you read reviews of black vocalists from the teens through the thirties you will see the accent referenced a lot. Try finding old reviews of Billie Holiday.