It makes sense that there would be black people in Hogwarts considering London, a port city, would have a lot of travelers passing through it. And even in the 19th century, London had one of the largest amounts of black people in the U.K.
It makes more sense that there are black people in Hogwarts than if there weren't any.
And when you look at history, African-American people, especially in the 1800's migrated to Europe, especially France to get away from the prevalent racism. And if you look at a map... you see how close the countries are?
Pick up a history book, dimwit!