Obviously there are accents that exaggerate certain inflections and so on, and it seems to me that there's a certain amount of affectation in the accents typically heard in rap music, but the accent itself is "black" in the sense that it is either based on or mimics the accent acquired among the American black population from its overwhelmingly southern roots, through its subsequent history to today. It isn't purely southern or purely anything else. It's like many aspects of black American culture: it takes elements of the surrounding (historical) white culture and adds its own flavor.

Consider "black" churches. They have characteristics that make them uniquely black, but they are rooted in the white protestant norms that dominated the surrounding white culture at the time of their inception. The accent we're talking about wasn't manufactured for a genre of music. It is an iteration of the accent that was typical among the people who invented that genre, and those people were black.