A mutation of a gene isn't something new. It's an abnormality of a former version. Black's don't have that gene but have the base trait that could mutate into said phenotype. I'd wager Blacks have much more variety and possibility in their gene mutations than a White person...
Black populations in Africa are genetically more diverse than anyone else in the world, true. But mutations are new. New combinations of code produce brand new traits that did not exist before.