I must admit I've never read the book. I have a vague understanding of what it's about. I did check out some passages this morning and replaced the word with 'slave' and the intent really doesn't change. I think everyone knows who the slaves were.
This change was suggested by a Twain scholar who tired of seeing the book not used in classrooms. I just have this vision of say, a white kid being asked to read a passage aloud while sitting next to an African American kid. Regardless of the context, that makes me queasy and I can see why a school board, a teacher, parents of both kids, might not want to 'go there'. If this brings the book back to the classroom while maintaining the gist of it, I don't see the harm.