The Africans who arrived here as slaves may have been captured and sold by Muslims in Africa, but most of them were not Muslims themselves. They were mostly animists of various kinds, and the English colonists in some slave areas were actually very resistant to letting Christianity be preached to them, fearing that if they were baptized, they would have to be treated like human beings.
Blacks are completely misguided on their “Muslim past,” but heck, it makes a convenient recruiting tool for Islam.