Reading these boards I see quite a few claims that airlines are known to maintain and share secret lists of information about pilots among themselves, dirty facts that are excluded from PRIA and the usual background checks associated with pilot applications. This type of list is commonly called a "blackball list" and it would be filled with alleged facts that are illegal to possess by federal law, but still desired for making hiring decisions. As illegal information such lists would have to be kept secret and their deniability would be maintained. But do they exist? How do we know they exist?