I'm not sure but I doubt that Islam was the ancestral religion of most slave descended Black Americans, but it's being sold to them as such.
I don’t think the sources of slaves at the time had much exposure to Islam (though they do now); the slaves certainly didn’t mind eating pork, and spoke no language related to Arabic (and the Koran can’t be translated). Also, the actual taking of slaves from Africa ended long before slavery was outlawed.
Malcolm X actually lost some of his racist zeal when he went to Mecca; until then he never realized there were white Muslims (he met them from Turkey, Bosnia, Albania, etc.), and they didn’t give him the Jim Crow treatment. After he returned he knew blacks were the source of many of their own problems, and told them so.