Actually, Paul Robeson did play the role of Joe in the movie version of "Showboat"... the 1936 version! Here's a page with a side-by-side comparison of that Universal Studios picture and MGM's reworking of the show in its 1951 movie: http://www.geocities.com/TelevisionCity/Studio/8849/Moviemusicals/Riverboat/oleman.htm#stage

Also, here is an obituary of Mr. Warfield: http://www.dallasnews.com/obituaries/stories/082702dnovewarfield.5c2a0.html The cause of his death is not known, but there will be an autopsy. He died at the Rehabilitation Institute of Chicago, where he was recuperating from a fall at his home.