Interesting query! The National Park Service actually has a small essay on this:

http://www.nps.gov/ethnography/aah/aaheritage/FrenchAmA.htm

Essentially, Louisiana, which as we all know started off as a French colony, had a large African population as slaves (it also had a sizable 'mulatto' population, but the status of half-blacks and full blacks degraded rapidly after the region was purchased by the United States), so it's not surprising many African Americans today, particularly those descended from black Louisianans, would have French names. :)