There are many reasons players produce black screenshots, it doesn't have to mean they're using hacks. It could be the configuration of their set up, anti aliasing.. any number of things which is why its not a good way to decipher if someone is cheating or not.
There are codes that appear on screenshots and can be explained 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).
Thats not to say there aren't cheats out there that exploit the above flaws in the system. At the end of the day its the server admins decision. I would say if they're banning you purely on that kind of evidence, find a different server to play on :)