In fact there has been black African colonialism - although it was practiced mostly in pre-European colonial times, as various African empires fought against (and often enslaved) one another - intra-African slavery being largely based on tribalism and tribal conflict. Europeans didn't invent black African slavery, although Europeans did take it to a horrific new level that anyone whose ancestry is from the slave-trading part of the world (certainly including mine) should be ashamed of. Africans in general didn't "colonize" or "oppress" Europeans simply because of a power imbalance that was present when the two cultures collided. Had power been out of balance in the opposite direction, it's likely that there would have been African colonization of Europe, and who knows, maybe even white slavery, with Europeans being kidnapped and taken to Africa. Some alternative history possibilities there.