You knew I had to jump in there, didn’t you?
I thought about that too, for quite some time, but both UV- and GitD-ink are incredibly toxic… I think no serious tattoo artist would inject that stuff. It exists, but it’s more of a “fashion gag”, in a way. The stuff is not healthy, and there are no real long-time-tests on how this ink behaves inside your skin (considering fading and all that).
This might be more important for a QR-code, I get that, but you can keep tattoos looking young and fresh - choose a really good artist, take good care of your tat and avoid sunlight. Like I said, I know someone whose tattoos are more than 20 years old and still look like they were done a month ago or so. I think if you make the QR-code big enough, that should really be no problem - blurring lines are more a problem with tattoos that are too small, like tiny text-pieces.
I know you won’t get one at all, so this is more of an advice for those who actually think about tattooing a QR-code on their skin 