Muslims have used black Christians as slaves for centuries, and are still doing this practice even today
Slavery is deeply embedded in Islamic law and tradition. Although a slave-owner is cautioned against treating slaves harshly, basic human rights are not obliged. The very fact that only non-Muslims may be taken as slaves is evidence of Islam's supremacist doctrine
https://www.thereligionofpeace.com/pages/quran/slavery.aspx#hadith