This occurs to me pretty frequently when playing! For now the fix I have found is to pause and unpause the game a few times, usually does the trick. However it would be nice to see this fixed!
The screen will get dark at random and requires the game to be paused (ESC) to fix. I can't say I know what makes this occur, or what maps it happens on, but I do experience this quite often.
Yeah this is quite an annoying problem. I think its some sort of dynamic lighting issue. When you walk into a dark area or shade the entire brightness of the game will start to lower. Its cool and all but the problem is once you step away from those dark areas the game doesnt readjust the brightness leaving everything pitched black even when staring at the sun.