I'd have thought "big black sailor" would sing better than "big black seaman", Brian, (and of course that's a word only too likely to set some giggling anyway).

Songs change as language changes, and they always have. "Big buck nigger" doesn't mean the same today as it did back in the 19th century. It has a shock effect which was absent then. It would have sounded then pretty well the same as "Big black sailor" would today, and conveyed to the listener the same picture. So it's a very reasonable substitute, in many settings (not all).

I wouldn't carry this over into written literature, but that's because we have a different relationship to songs than we do to written literature.