Shared needles.
I don't think that's the case. I think it's black men having sex with other men ("down low") and then passing the disease onto their girlfriends/wives/friends with benefits. I couldn't find a CDC chart with transmission categories subdivided by race and gender, but I did find a chart stating that almost 85% of the HIV cases diagnosed in female in 2009 were from heterosexual contact.
I haven't seen data suggesting that blacks use intravenous drugs at a higher rate than whites, yet according to the data, 46% of black MSM (males having sex with males) are HIV-positive. Studies show that black homosexuals are less likely to use condoms when engaging in homosexual sex and their unsuspecting partners then contract the disease when they are engaged in heterosexual sex.
Contrary to what the media would have everyone believe, HIV is still largely confined to small segments of the population (intravenous drug users, homosexuals, and heterosexual women that have sex with homosexuals). It is nearly impossible for a heterosexual male to contract the disease from having regular sex with an infected female, yet if you watched media coverage of it, you would think that there are a lot of guys like Magic Johnson out there.