Blacks have stronger bones? What a joke. Caucasians have always dominated strength sports and almost always weigh considerably more at roughly the same size. Even with a black's generally-less bodyfat percentage, this does not fully explain such a disparity. Northern Europeans are stronger, thicker and more densely muscled.
The voluminous muscularity of the average black bodybuilder should not be misinterpreted as being bigger. Rounded muscle bellies, lower bodyfat, natural tan, and balsa-wood frames/small joints contribute to success in physique competition
Know what you're talking about.