I'm skeptical that the US would get that involved in Africa as a whole. The Congo Free State is very much the exception in terms of the public outrage it generated, and it's worth noting that a lot of the other colonies were doing similar things and were only somewhat less brutal. Plus, as
@Japhy notes African-Americans didn't necessarily feel solidarity with Africans, so in a lot of cases they won't be much more concerned about what's going on than white people are. The vast majority of invading and slaughtering that Europeans probably wouldn't bother Americans too much. The other factor is that the US isn't going to want a trade war with the major powers. Boycotts and embargos against Belgium are easy, but doing the same thing to Britain or France would lead to retaliation and that would seriously hurt the US economy.