I've noticed this too.
Black guys (people of Arfo decent) tend to have noticably better genetics than white (European) people. I believe that this is from natural selection: until about 100 years ago, in order to live long enough to procreate, Africans would have to be quite physically capable, weaker genetics (in terms of physical strength and size) would generally not survive.
Whereas in Europe, with the "advent" of the feudal system and much more structured society existing for a few hundred generations, the need to be physically strong is not as important. People with other qualities were able to do things which allowed them to florish and find a mate. Wealth and medical science (however basic) allowed small, weak, fat, people to procreate and therefore, white Europeans are much less likely to be of equal genetics to their African cousins.
As an asside, although African skin seems to be a bit thicker, dark skin looks better for showing definition. This is evidenced by the fact that everyone tans as dark as possible before a show.
(but dont forget Rhul, Priest, or the fact that Dorian dominated in a very competative era!)