has there ever been a black worlds strongest man winner?
Of the MET-Rx contests? I don't think so.
However, that title was given to one Mark Henry, whom FLEX Magazine ranked as the 2nd Strongest Man of all time.
Henry has set world record in powerlifting and won the very first ASC Strongman competition in 2002, defeating a number of MET-Rx World Strongest Man competitors (which, I think, included a couple of MET-Rx World's Strongest Man winners).
Edit - Mark Henry did defeat an eventual MET-Rx World's Strongest Man Champion, 2006 winner Phil Pfister.