How far back do you wanna go? I have been reading alot lately on the Oromo ins southern "Ethiopia". Did you know they also alledge slavery still exists in modern Ehiopia? Some would refer the Oromo as "Ku****es" even though they are the earliest "Ethiopians". The "Ku****es" in Sudan have, since the time of the conquest of Ethiopia's King Ezana fought against conversion to Christianity. Many Oromo and southern Sudanese still practice indigenous religious systems and the Islamization process is came later after the Ethiopian Christians moved south and westward to subjugate these remaining "Kush" ites.
http://www.africanholocaust.net/news_ah/ethiopianslavetrade.html
While "Arabs" share their part of the blame lets not continue to dismiss or overlook how long various AFRICAN slave trades have been going on. The Dutch and Portuguese were only able to establish trading centers on the coast of west Africa AFTER they destroyed the east African centers of Sofala and Kilwa, breaking ties of this African trade that extended to Angola and over across the Indian Ocean up to China.
Darfur is the remnant of this ancient system of oppressing Africans who attempted to hold onto their indigenous religious systems and ironically the Christians in Sudan are the remnant of the same Axumites that destroyed Kush.