The status codes for black screesnhots are as follows:
B0 = Application not active screen capture failed.
B1 = Screenshot successfully attempted line present.
B2 = Screenshot successfully attempted line not present.
B0 - The game is minimized or in the background.
B1 - Successfully received. Can sometimes return a black SS.
B2 - Is caused by one of many reasons (drivers, hardware, bad connection, etc).
Anti aliasing can cause black screenshots. It seems to be a vista/g-force anti aliasing issue on one of my computers. Try Ice mans suggestion and see what happens.