I don't have this problem on LFS, but I have on Just Cause 2 and some other games (DiRT 2 too).

It is very random. Some games work wonderfully whereas others just don't take printscreen (I get a black screen).

I have experienced this in LFS, but not in other games. It's not much of an issue anyway.

Well, nobody says is their version of W legit and what specs they have, so we have no clue is it random or only on some systems, also, we got no solution for it...
Have you tried google or the program I found, ScreenGrab?

Well, I've never cracked windows and I've seen the problem before quite a few times on various versions, as I said above.
I don't have the problem in Vista on my laptop or W7 on my PC afaik.

Well, I found something about possible fix of getting black screen while taking print screen in game.

http://think-games.blogspot.co ... f-games-in-windows-7.html