In today's Parade magazine a new Sinatra book is being published. Denzel Washington would be the perfect title role character with his sidekick Dean Martin played by Cuba Gooding Jr.
Fair is fair. So why wasn't Tom Cruise tapped for the title role in Ray?
What we are dealing with here is "the relationship is central to the story".
Kojak or Bosley being black or white was never "central" to the story. Any ethnicity could play those roles.
The fact that Huck was white and Jim was black had everything to do with the story between them.