Thank you, Acorn4, for the link to the "Showboat" article.

My view of this is that if the song is being sung within the performance of the play, which is set in a period when the word had currency and it should be performed as written.

On the other hand, if it is being sung apart from the show, then the word should probably be euphemistically replaced.

As to Hammerstein's purposeful use of the word, it was not the only time he used his pen to point out racial injustice. "The King and I", and "South Pacific" both had race as a background.