Oppression is nothing new, and it has caused suffering that echoes across generations. That one group was oppressed does not mean that members of the oppressing group--or at least their descendants--benefited from it. In a post with which I largely agree at Sojourners, Soong-Chan Rah laments "The American Church's Absence of Lament." Professor Rah goes astray, however, by lamenting "the reality of suffering and pain from which many of us in the United States have benefitted (sic, emphasis mine)."*
Specific individuals benefited from slavery, colonialism, imperialism, etc., but this doesn't mean that entire societies (or large chunks of society) benefited from them. Slavery, Jim Crow, and other oppressive institutions mean not only a lot of wasted time and energy maintaining those institutions but also the waste of a lot of human capital. What inventions and innovations do we not enjoy today because the inventors and innovators were kept in bondage? What cultural treasures do we not enjoy today because many millions were kept illiterate? Here's Wikipedia's "List of African-American inventors and scientists." If we restrict our focus to oppression of African-Americans in American history, we've lost quite a lot.