I'm going to give it another shot, since I just got past this point in my replay of the game and I think I can link the riddle to it. It's a bit more small scale.

Is it Mayor Domino when you're infiltrating Shinra HQ?

Would-be small scale vandalism - Some books in the library are mixed up.
At times has large scale impact - Holds the key to the password that you have to give the mayor.
Black pawn attacks not black king - He's just a figurehead (pawn) for Shinra (King). He doesn't directly attack them, but...
But may abet the white foray - He does give you the keycard to get further into the HQ.

Again, on the chance I am right someone else can take my go. It's just this riddle is driving me nuts.