Having black be the transparent colour would be dumb.
Imagine a fence or grate. It is vastly more likely that part of the actual structure will be black than it is that it'll be pink.
You won't have pink fences in Quake, right. Black ones? More likely.
It's ideal to use a very unusual colour for the "transparent" job.
Edit: You're from Libya? Then I'm surprised that you have internet atm. Didn't your Dear Leader try to switch it off?
I'm from Germany, and we're pretty interested in what happens in Libya. Good luck!