Greg Ellifritz over at A.R.T. made a comment on this that I agree with 100%: Regardless of what type you have written/ patched/ or stickered, the ER is still going to type your blood before administering. Simply to help avoid that liability. They have no way of knowing if you borrowed the boots/ jacket/ body armor bearing the blood-type patch. Hospital, blood typing is apparently a pretty routine and swift process.
This is a classic example of a piece of practical military warfighter kit that migrated over to the civilian/LE world where it does not have as much validity.
