I think most of it lies on the photographer....if Inez, both Stevens, Mert & Marcus and Testino made a point of shooting black models there would suddenly be a big following. Steven made Beverly Peeles and Naomi.
I'm constantly hearing from clients I'm working with that there's no blk models out there.....what they really mean is that they don't want to use a commercial girl. Black models have to turn to commercial work to support themselves....the min they do that they are out of the editorial game. Then these brand new black models that are babies, show up and haven't had anytime to "get it" for editorial work then they are out. Look at Missy, Anya and Racquel those girls were able to develop into what they are.....white models are always able to re-invent, black models don't have that option.
So if the blue chip photographers would chose several black girls as muses as they do with the white girls then you would see more of a diversity.