I'm not surprised at all that Stanford has the highest percentage of black students at 7.5%. I haven't done the analysis, but I'm willing to bet that their racial demographic generally mirrors that of California as a whole. I am surprised with the relatively low percentage of black students at USC.
Stanford is one of the very few schools that can basically choose it's enrolling class. It's had the most selective admissions in the country for the last few years. They really do say, "OK, now we need an oboist, coxswain, and a transgender student" and send out the acceptance letters.