Hmmm...well, if Europeans happened to have darker skin than Africans - by your reasoning Africa would have colonised and oppressed Europe. Nothing to do with the colour of ones skin, but rather the colour of ones thinking. Europeans arrived in Africa and were met by other humans who had no means to resist Europeans with advanced warfare and greed for resources. 
The colour of Mngxitama's thinking is the pitch black of evil and nothingness.