Sisko is very aware of 20th century America's love affair with racism apparently. Presumably he is also aware of how it grew that way from the 17th century up to the 20th century. He doesn't seem to say "wow, what a shitty time to be an Africa, but it does get better in the long run and now we, in the 24th century, no longer have ethnic strife among humans."
If the writers had really wanted to look at the issue, they could have very easily put Sisko in such a position that he was forced to be captured as an African (from Tribe A) and sold as a slave, been the African capturing other Africans for the economic survival of his tribe (Tribe B), and a missionary decrying slavery on moral and economic grounds, and even a merchant who was for slavery simply for his own bottom line, all of which actually existed. Let the writers put Sisko through the wringer so he can see it was far more than just the simplistic "but racism is bad" angle. Or make him experience Reconstruction and suddenly go from having someone at least provide for his needs of food, clothing, and shelter to being turned out onto the streets and told "good luck Darkie" and have to figure out how to provide for Jake and Cassidy in 1866 Louisiana.
Look at Sisko's commentary on going to Vegas in 1962 to Cassidy. It's about how blacks are treated and "our" people. He shouldn't even think of that stuff any more than I think about life in London in the 1500s for my ancestors, but again the writers aren't writing to serve the characters but for their own Baby Boomer sympathies about how tough these people had it in the past but how we can't move forward because of just how painful it was. Even Cassidy tells Sisko that was the past and they aren't bound by it, in effect "it sucked for them, but it ain't us these days so stop bitching about it."
And while we're on the subject, even changing the ending of the last episode of DS9 so that instead of Sisko effectively being dead because Brooks didn't want Sisko to be an absentee father in the form of a MIA war casualty, they made him an absentee father who could come back any time but so far still hasn't, because somehow going out for a pack of smokes and never coming home is somehow more noble than telling his son "your father died a hero". The writers thought they were doing something for Brooks's message and ended up writing the thing they weren't supposed to be writing at all and even Brooks didn't get it.
