The film is set at the end of of the Civil War right? The word had not lost popularity by that time seeing as how M Twain was a big fan and wrote his works 20 years later and Agatha Christie (a brit) would use the word in a famous book title almost 70 years later in England.

I think that between 1856-1858 the word would have been commonly used among most. Especially whites.

This isn't defense of Tarantino