So, you don't mind identifying yourself as an African, but you don't want to be called Black? The reality is that dark-skin is a trait that is most associated with Africans, even if all Africans aren't dark. Same thing with kinky hair, full noses and lips, etc. All people of African descent don't have these traits but they're still traits you most identify as belonging to them.
Point being?
Your argument makes more sense with Buffy because she's a model and video chick. Her "look" is what she sells. The male pornstar ain't really selling his look. A couple, like Mandingo and Lex, may be selling their dick size, but that's about it. It's the females and/or the genre that sells porn.
Do you equate dark-skin with "low-life"? As another poster said, you must believe in "degrees" of blackness. Porn exists to cater to the fantasies of its white male fanbase, and a dark-skinned big-dicked buck nigga is part of their fantasies.
Now, you could be on to something here, in that Black America has a color caste system, so light- and brown-skinned African-Americans are more likely to be middle-class, go to college, hold professional jobs, etc. and dark-skinned African-Americans are more likely to be in the lower-classes, work menial jobs and be incarcerated. And, assuming that porn tends to employ lower-class people, then it will attract more dark-skinned people. But, this is highly debatable and I'd have to see some empirical evidence to support it.