I'm not so sure about the history of Africa before the Arabs spread Islam, but Islam could be a reason as well. But also it could be anger and issues of who's more powerful. Is there a religious implication to this, as to why so many of these countries are stuck in a vicious cycle of violence.
Why would you equate black Africans killing each other with Islam being a reason in them doing so. Firstly your use of term 'black' black is not a race if you are intending to mean people of the negroid race which is mainly prevalent in west Africa, then west Africa is majority Christian and west African Christians have killed as many Muslims or indeed fellow Christians as Muslim west Africans have. If you are including North Africa and East Africa which are both majority Muslim but equally not of negroid DNA therefor could be subjectively defined as being 'non black' equally kill more Muslims than non Muslim minorities. You appear to be ignorant, racist, islamophobic and irrational all wrapped into one.
Categorically you cannot blame one emotion and one religion as reasons for people killing one another. Whether or not "black Africans" kill each other more than the norm is another topic up for debate. What are you specifically talking about? The subjugation of Africans via religion, Rwanda? Darfur? South Africa?
Tribes of blacks were selling other blacks they'd conquered to whites and Muslims LONG before blacks took on Islam in Africa themselves (those that have taken it on, that is). They were warring among themselves for territories, much like some American Indian tribes, and what better way to ensure no retaliatory attacks than to ship them off to other countries?
Severe poverty and lack of basic resources does ugly things to people of all races, creeds, colors and religions...