Are all Black people mixed because of slavery?

They claim all black people are descended from Africans and yet black people in America, Jamaica, Russia or any were in the Caribbean don't look like Africans. If you don't believe me, then search the internet for pictures of black Africans and the look for pictures of black Americans and you'll see that we don't resemble them. I have heard some people say that is because all black people are mixed, but truly most black people procreated with each other during the slavery period and even though black slave girls had children for white men either by rape or willingly sleeping with their white slave driver, blacks still tremendously outnumbered whites on the plantation. So there is no way they could have caused all of us to be mixed in just a time span of 4 generations. So what do you all think?