It doesn't make a lot of sense to me. If you donate blood, they do give you a card. In times of scarcity this is supposed to guarantee you access to blood from the blood bank ahead of someone who wasn't a donor.
But it's not transferable to other people that I am aware of. I am not sure what collecting cards from other people would do.
If you need 3 pints of blood during some surgery and you have only donated 2 pints, it's not like they are going to refuse to give you what you need.