The RFID solution is to embed a tag into the blood bag label itself. The paramedic who transfuses the blood can scan the bag before transferring. He/she typically enters the patient ID number, or in a better system, the patient also has a wrist band RFID tag which identifies him uniquely.

 In case the wrong blood bag is scanned, the reader can throw up a warning like this :

This will save the life of Mr. John Smith (who will no doubt, be eternally grateful to the technology, if only he knew what was about to happen!)