The thing that reduced the black population in Arab countries was the practice of castrating all Male slaves - yes, exactly.
The Arabs bought black boys from black slavers. Then, they castrated the boys. The eunuch slaves couldn't have children and were thus deemed safe to serve Arab households.
The Arabs didn't bother with things like Western medicine or anaesthetic, they just lopped off the black boys' genitalia - testicles, scrotum and penis - just like that. Many boys died because of this awful, inhumane procedure. The fatalities of the Arab slave trade dwarfs that of those slaves brought to the Americas.
But the Arabs had thought of this. Don't worry, they had it all worked out. They were rich and they simply bought many more black boys, and lopped off their genitalia.
Some survived.