I think this is actually a good question, and I don't know the answer offhand. I know HIV originated in Africa, but I don't know why it caused so much more damage to sub-Saharan Africa than to other similarly impoverished nations (e.g. South Asia). As for American blacks, I'd need to see this controlled for income level, but I suspect a huge gap will still remain.
Here's Wikipedia's
list of countries by HIV/AIDS rate. I'll assume it's roughly accurate and analyze it by region.
The highest HIV infection rate is Swaziland at 26%. The top 24 countries are all in Sub-Saharan Africa, and a number of countries in the Caribbean with large Afro-Caribbean populations (e.g. Bahamas [#25], Jamaica, Haiti) rank quite high too. The highest proportion in Asia is Thailand at 1.2%; all other Asian countries, regardless of poverty, are below 1%. All Spanish-speaking American countries are below 1%; the worst is Guatemala at 0.8%, which isn't far from the USA at 0.6%.
In Europe, rates are below 1% except in Estonia (1.3%) and Russia (1.1%). I'm not sure what's going on with Estonia, but Russia is seeing its HIV rates rise rapidly due to spectacularly bad policy. Social taboos on homosexuality combined with anti-gay laws, no needle exchange centers or methadone maintenance, high injection drug use rates, and lack of concern about HIV by the health authorities is a perfect storm for an AIDS epidemic.
Overall, though, I can't find a good reason that HIV would affect poor black populations much more than poor populations of other ethnic groups. Also, I don't know why it hit southern Africa the hardest, while West Africa isn't as badly affected. I suspect that there could be something genetic going on that makes African populations, and eastern and southern ones in particular, much more vulnerable. Does anybody know of any good info on this?