I've heard that too, why though? Counterfeiting?

You can fill the paper out with whatever you want and it is already signed by the player. Could cause some legal headaches. Next thing you know he has signed a paper and you filled it out That REDANDGOLDPRIDE INC is his new sports agency and he owes you 10% of his contract! I know that is an extreme example, but things like that is why they are being told not to sign blank papers.