Just wanted to respond and say that when mine goes black, its completely black. You cannot see the game at all except for an in/out flickering of the actual UI.
Also, it seems to be related to the arrow that appears when you level up. I've worked out that if I leave a co-op game, and someone else levels up my guys, opens all the inventory and gets rid of the yellow arrows, I can then come back in to a normal game.
