Well, if a client side add-on also, any color flash is possible.
I believe the only way to do this without being able to mess with the source code is to make a full screen GUI Bitmap whatever that would probably disrupt everything with a slow fade. It'd be exploitable and broken. The point of his request was for cutscenes and such, likely for people joining spontaneously. I'd defeat the purpose of the Add-On to force people to have it just for a simple effect. I really don't feel like trying to explain, I'm failing.
BlackOut is a client command, WhiteOut is server-sided, from what I can remember.
They're both clientside, and the server can tell their game to do it for some reason given. Although I think there is some minor differences with them... I'm, confusing myself, nevermind.
You described a flash bang.
The forget are you even saying? Who described what? He's asking for a black screen overlay and you're going on about those? I hate you.