This is a very complex question that requires you to take alot of things into account.
The biggest of which are TV contracts which are usually renewed by the decade. If the NFL has a TV contract with CBS, FOX, NBC, ABC (espn), etc, that's not going to change just because of a mass exodus of black athletes.
However the endorsements and the money behind that would follow the player unless their contracts have clauses that say they must be employed by the league (some do, most don't to my knowledge).
But you have to worry about other things like stadiums, travel, city permits, etc etc. Things that can be established over time but not overnight.
As far as cacs watching it... well there would be nothing to watch if it wasn't on TV due to existing contractual agreements. But lets say it was on TV. The biggest result would be a massive... MASSIVE dropoff in how many people watched sports period.
Some cacs would watch it because they enjoy the sport and want to see the best athletes
Some cacs would watch the old leagues out of nostalgia, tradition, coverage, etc
Some cacs would not watch it out of principle and end up not watching anything
However, down the line I think that NFL and NBA would eventually become MLS status at best if no black people played in those leagues.
However i think the MLB would be just fine.
I dont understand why this isnt discussed more often? At the least...large corporations would still seek out money partnerships nd deals...or would our athletic talents/skills suddenly be "not a huge deal now? 
..."They just put a ball in a hoop or a goal"
exactly
