As a few posters have said, there are many, many, african-american chicks in the porn game. Top ones right now are:
Roxy Reynolds
Nicara (Aurora Jolie)
Melrose Foxxx
Erika Vuitton
Marie Luv
... and that's just a few of the headliners with recognizable names.
It's true, quite a few do fall into the "niche" markets someone mentioned; you're not going to be able to avoid that, unfortunately. However, thanks to sites like Newbie Black and Round and Brown, many of the new starlets that break into the business often hit those sites first. Exploited Black Teens is another good site to view, as well as Bang Bros.
Chicks that have "white" features (Marie Luv, Nikki Fairchild, Lacey Duvalle) have an easier time "crossing over" and showing up in the better studios.