LOL, @ all blacks being Muslim. Although in the 50's and 60's the Islamic movement was gaining popularity in America(with people like Malcolm X and Muhammad Ali among them), it later fizzled out. I think that certain Black Islamic figures have made it seem like Islam is more widespread then it is. No doubt Louis Farrakhan (a widely known muslim leader) leading the million man march back in 1995 may have given that impression, even though only a few thousands Muslims were actually involved in it.
That said a large group of Africans are in fact predominantly Muslim. Especially northern African countries where you can find countries with over 90% Muslim populations.