It used to be a thing in some military armies in the last century, to facilitate blood trasfusions in case the person was unconscious.
Both my wife and I have such tattoos - she's B+, I am 0-. As an aside, it's a false myth that tattooed people can't donate blood - I got lambasted for getting tattoos because my group is universal donor and in the opinion of this person I was being "selfish" because it would have prevented me from donating blood. In reality, both my wife and I donate regularly.
