"but we sure he's not black"
They may assume he's French, but beyond that they wouldn't have a clue. Just like if this was a musical about Alexandre Dumas fils, most would assume he was French by looking the name, but how many know that he was biracial? VERY very few. So in casting that musical you could cast a white actor or a black actor in the role and very few would care in the least, either way -- unless the show dealt with how his race had an impact on his life and work, at which point you would need to cast an actor who looks black or biracial. SUNDAY doesn't deal with race, so the race of the actor playing George shouldn't matter.
I've seen black actors play all sorts of characters which were traditionally cast with white actors, from seeing Keith David play Sir Walter Raleigh to Audra McDonald winning a Tony for playing Carrie Piperidge in Carousel (a New England woman from the early 20th century who would presumably be white if one were being historically accurate) to Brian Stokes Mitchell winning a Tony for Kiss Me Kate (playing a touring classical actor -- aside from Paul Robeson touring Othello in the 40s, no black actors toured in leading roles with otherwise all white companies during that period) to dozens of black actors playing all manner of English kings and queens, Italian noblemen and women and other Europeans in Shakespeare and other classical drama. And you know what? No one cared about the race of the actor or thought that it somehow detracted or interfered with the production. Great actors can play almost any role and create sufficient suspesion of disbelief to make an audience believe them in the part and forget any and all notions of historical accuracy or "academic fallacy."
In short, aside from the occasional narrow-minded observer, audiences have proven time and time again to be able to accept nontraditonal casting in all sorts of forms and actors of various ethnicities have had hundreds (thousands) of major triumphs playing roles that moulinrougehk would have denied them the opportunity to play.
"What a story........ everything but the bloodhounds snappin' at her rear end." -- Birdie
[http://margochanning.broadwayworld.com/]
"The Devil Be Hittin' Me" -- Whitney