As documented in the book "The Slave Trade", it was the Muslim Koranic requirement for Purdah, or seclusion of females which practically mandated that black eunuchs be employed. Thus, the Trans Sahara Route, to Muslim Cities in or near Arabia prediminated. Since the castration of the male slaves took place at a waystation in the Sahara, there are no indiginous populations of African Blacks anywhere in Arabia.
For the vast majority of those who took the Trans Atlantic Route were never subject to emasculation, we have thriving African populations throughout the New World.