I note that Steve Parkes mentioned "Showboat". Has anyone ever heard the original (1929?) recording that Paul Robeson made of "Ol' man river"?..
The original first line is "Niggers all work on the mississippi". Most people will know Robeson's history in black civil rights. He obviously felt that in the context of the period that the story is set that these words wore appropriate or he would have refused to have sung it.
For the film in 1933(?) Robeson re-recorded the song with the words "Black man works on the Mississippi" but I have to say that the original, to me at least, is more effective in reminding me of attitudes to black people at that time.
To say that there are no times when you can use these words is just not true.