"...why does a black midwesterner sound much like a black southerner when a white midwesterner sounds very different from a white southerner?"
"Ghetto" (Snoop Dogg) and "Country" (Charles Barkley) and "Creole" (Beyoncé Knowles-Carter) are three completely different African-American dialects and social cohorts. But a Cajun ordering French dressing in Bangor is like an Abbott & Costello skit.
Several of the "other" 19th Century North American languages never really went away:
Quebec... 'nuff said.
Gullah-Geechee south of the Cape Fear down to Charleston or so.
Acadian-Cajun on the Gulf Coast (and the Maine Maritimes, sort of.)
Tejas, Nuevo México and Arizona will probably always be multilingual. (You don't hear much German or Bohunk 'round Luckenbach these days though.)
Cantonese & Hokkien (in eleventeen dialects) in San Francisco and Los Angeles.
And of course Hollywood, Broadway and any deli worth eating at run on a good bit of Yiddish.