Some people rise above the circumstances of their upbringing, some never do.Imagine ML-King became president of USA and 15 years on another black man became president and was implicated as well as his cabinet in a number of corruption scandals with no real repercussions following, would you say that he is sinless because of having a previously disadvantaged background?
(Though I wouldn't call the latter "sinless"...)
Besides, remember, you're comparing Africans & Americans. We- or at least, large subsections of our country-have done exactly that with old white racist politicians in the past.
It is going to take time- possibly 2-3 generations- before you're going to see a consistently better kind of politician rise up out of the oppressed underclass. Not just because it takes that long for those people to become worthy of notice, but also to have the electorate recognize that those guys are better than the demagogues. And even then, you're going to see spikes of nativism, racism (veiled or overt), isolationism, and all those "isms" that hold nations back.
Exhibit A: look at what is going on in American politics right now.