Is there a word like "colored" or "darkie" that would be offensive to a white southerner during the Civil War? I don't think the N word would work here. I'm working on a screenplay and want a southern mom to be offended when someone calls her slave a "darkie," but, sadly, I don't think anyone white would have found this word offensive at the time--so I'm looking for a different word.
I don't think any of our answers are helpful. The unfortunate fact is that if you want to capture an era, you have to steep yourself in that era. The only way to do that is to read widely and deeply -- Faulkner and Angelou and Baldwin and even (gasp!) Mitchell (inheritors of that era) and some diaries from that era. There is one in particular that I read a review of, or a comment about, recently -- it was called something like "Diary of a Charleston Lady", but you should get the more recent, unedited version -- the earlier version was extensively edited (whitewashed ?) according to the review or comment, and thus was far politer than the original. According to the review/comment, present day Charlestonians love the edited version, but not the unexpurgated version. Sorry not to give you a detailed reference, but I just don't have it.
There are a number of slurs that have been used throughout history. The hardest part, I think, is making sure the term is period-specific. I know people who used "Apes" "Ace Of Spades" or just "Spade", along with others.
I found a website which lists slang words, but it does not give the era in which they were used, so I guess you could look at them and hit up dictionary.com to find the origins and times. http://www.rsdb.org/race/blacks
Hope this helps! -- John