I just started listening to the New Books in History episode interview with Patrick D. Bowen on his new book, "A History of Conversion to Islam in the United States, Vol. 1: White American Muslims Before 1975" (Brill Academic Publishers, 2015) The interview and the bibliography might be a good entry point for you.

But the short, historian, answer is... it depends. An unknown (to me, at least!) number of enslaved west Africans in the early 19th century were Muslim, but as the slave trade ended, so did new forced migrants to refresh the population. Muslim traditions may have been sustained within a context of evangelical Christianity in some enslaved communities, but that's real hard to discern and by emancipation, religious life among African Americans was almost entirely Protestant/Christian. In the second half of the Nineteenth Century, of course, diaspora communities from the Middle East migrated (particularly from the Ottoman Empire) but not in the same volume as Jewish immigrants from eastern Europe. Some native-born white Americans in the last decades of the 1800s took a very liberal, esoteric, approach to pan-religious movements that included a healthy respect for Islam...which is what I understand Bowen's book to be about.

This is all off the top of my head, so I'd be happy to be corrected!