They bought more black slaves than Euros did. However they castrated the males so they could not reproduce. That's why there is no black diaspora in those Arab countries.
Are you talking about the Janissaries of the Ottoman Empire? Not all were castrated.
Slavery in Islam is very different than slavery in the Americas. The Koran says God smiles when a slave is freed. Plus, children born to slave mothers had rights to the father. Consider Prince Bandar.