Actually, it's kind of a linguistic relic of certain regions in England, from which the white people came from whom most US blacks learned English. The Scots-Irish (descended from Protestant English colonists in Ireland, who were imported to Ireland to build a core of loyalist population, and came from the border & highland regions of Britain) were the predominant settlers in Appalachia and the Deep South and a lot of behaviors we associate with hillbillys and rednecks can be traced back to studies of those people in England. A lot of what we think of as black culture, including the dialect & idioms (including terms like "cracker" & "redneck" ) come from those parts of England. The settlement of America was not "white people going wherever", rather ethnic groups tended to congregate in certain regions, even groups from different parts of countries that we think of today as homogenous. The Germans in Pennsylvania all tended to be from the same parts of Germany, and Germans in other places came from other parts of Germany. The whites of New England were from a different part of the country than those of the Tidewater regions of Virginia or what, in those days, was the Western frontier, and the cultures were correspondingly different. The Civil War was not just about political issues or slavery, it was a conflict of cultures within America.

In all of thise, blacks learned English, and adopted the religion and other practices of their white neighbors, generally the redneck types, rather than the planter aristocracy. Because talking like the rich folks was a good way to get shot or whipped or whatever. But for whatever reason, the language actually changed LESS in the American South than it did back in England (in fact, some people think the stereotypical southern drawl is a lot closer to how they talked 17th or 18th Century England, than modern "received pronunciation" ), and so black Americans still use idioms from the border fringe regions of 17th century Great Britain.