I used "African" here to identify indigenous folks basically below the Sahara and not identifying as "Arabs" or immersed in the Arab culture. If I recall correctly, Islam forbids Muslims making those who are Muslim as slaves, obviously some slaves convert but I don't know what happens to them I assume they remain slaves. The point is still valid, native/African person were taken as slaves for export by outside forces (European or Arab) and native/African persons were most often the forces that made the initial acquisition of the slaves who were then transported onwards to entrepots where they were sold to the outsiders.