So I guess the question is how long would you wait after Black recovers to blow the whistle?
I'd wait until one of these happens:
1. The A player realizes that the ball is live and advances.
2. The A player throws the ball to an official.
3. The A player takes the ball out of bounds.
This is a live ball and it is not up to the officials to decide to kill the play prematurely. Yes, there is the remote possibility of a team B player blowing out the A runner, but I doubt that. More likely there would be 9-2-3 fouls for substitutes interfering with the play.