As has been alluded to above on several occasions, what is considered offensive now was not necessarily considered offensive in the past; we might also assume that what is considered quite innocuous at the moment, may very well be considered offensive in the future. A good reason as any for not applying the blue pencil but just accepting what was or we'll receive a very skewed version of history. I remember from childhood 'black' being an offensive term while 'negro' was considered a polite reference to race.
Work, by the way, was against slavery and used the dialect in his songs which he heard the slaves using so considered it quite proper.
I don't know about non pc songs but how's this for a non pc singer:
'Trench Blues' sung by John 'Big Nig' Bray, recorded for the Library of Congress.