If I’m injured and need a blood transfusion, where are paramedics most likely to go for the needle; is it the vein in my arm, same as for blood donation?

Wherever it is, if I get a tattoo noting my blood type, are you going to trust the tattoo and give me my blood type, or would you disregard it and go with O- just in case?

I’m getting some tattoo work done anyways, and I really like the idea of a Canadian Blood Services logo plus my blood type wherever paramedics would see it, but it’s pointless if they won’t believe it