Wow, what perfect timing.
I am doing a report on censorship in group dynamics, and we will be on campus filming people regarding the issue. While doing research I found a ton of documents that were blacked out, as well as sites that showed how to read the blacked out material. I didn't visit any of them, but I imagine if you put black over black (black being the type), and white (being the paper), the black/black would still be darker than the rest. Unless you use a computer program to alter a document, or scissors, I don't think blacking out areas with a sharpie will completely do the trick.
Google "government censorship in media" and you should find the same links. (You can also go to the CIA site and read the recently declassified documents with all the blacked out areas.)