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One reason for the "secret mark" is that, back when blood banks were paying people for their blood donations (and it's possible some still are, at least for rare types), some unscrupulous types (frequently, but not exclusively, skid row residents) would try to get a bit extra drinking money by selling more blood than was safe. They were willing to take the risk to themselves for becoming anemic, but there was also a risk to the recipient of a transfusion because the second pint in such a short time would be less nutritive.
Additionally, nowadays especially, there are concerns that some people might be tapped, and then a lab test shows their blood is tainted in some way (hep, hiv, etc.) so it must be thrown out ... and this mark is an attempt to inhibit such a person (there was a very shortlived, very much condemned, suggestion many years ago from one - only one - gay activist that an effort should be made to spread AIDS far and wide throughout the straight population in order to stimulate public money for finding an AIDS cure - nobody would second the suggestion but it was enough to make everybody worry).
If you are donating blood, you get a (temporary) needle mark, a sticker for your coat, and expressions of gratitude. Why begrudge a (washable) dab of invisible paint?
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