Economic situation. There are more safe and less safe places to live, more safe and less safe jobs to have. Those with more money have more choice so avoid the more dangerous places. If you have less money, then you can't necessarily choose to say no to that waitressing job that gets off at midnight in that bad neighborhood, and you don't have a car to take you home, you have to wait for the bus on that scary corner. Also, I would expect rapists know that if you r**e a white woman, you'll get Nancy Grace on your @ss, but if you r**e a black woman, it just won't attract as much attention. And, even better, if you r**e a prostitute, no one will even care, the police won't pursue it at all. Plus if you kill a prostitute, no one will notice for months, if ever. That's why you see all these serial killers having killed prostitutes. That guy, Joran Van Der Sloot, who killed (it seems) Natalee Holloway, killed someone there was a lot of attention around, and now was caught before he killed enough (minimum of 3) to be considered a serial killer. But if you're Anthony Sowell, though, you kill drug addicted homeless prostitutes, and then even when the women escape and tell police and offer to take you to the home of the serial killer, the police do nothing, and you get to kill a whole lot more.
The same thing is true today as was true in Jack the Ripper's time--he killed prostitutes, but not just any prostitutes--almost all whom he killed were absolutely the lowest of the low. They were all sick, starving, weak, homeless, nearly friendless, and alcoholics. One of the victims would have died of disease within a week had she not been murdered first. The only reason he got attention for killing prostitutes (which was happening all the time anyway) was that it was done in a gruesome manner. The weekly murders of prostitutes in London didn't otherwise attract any attention.
Anyway, so, I wouldn't be surprised if a further study of which black women get raped showed that it was highly correlated to economic situation.