I don't think that "Darkies' Sunday School" is inherently racist - as long as we do something to get rid of that one word "darkies." There are lots of songs from the past that contain offensive elements. If you can remove those elements in a way that is not obviously prudish, you can end up with a good song.
"The Cat Came Back" is a good example. "Short'nin' Bread" is another, as are some of the Stephen Collins Foster songs. But if bowdlerize we must, we need to do it in a subtle manner.
-Joe-