"You tell no one. They cant do anything,"
I'm not so sure.
Prying eyes tend to sneak around. Lets say someone comes to visit someone in the hospital and discover that you are in there also. They realize the seriousness of your condition and realize you need blood. They can relay this to the blood police hospital liason committee.
Theres always weasels out there who know other weasels and wouldn't hesitate for a split second to 'rat' you out.