Death by Tattoo?

Closest thing I could think of was the X-Files episode "Never Again" but in that case there was ergot in the tattoo ink, which caused hallucinations leading to some serious self-harm.
https://x-files.fandom.com/wiki/Never_Again

I'm thinking that, given that people have been killed by a single pellet of ricin jammed into the skin, and that thallium and selenium kill over time, so it's not like the victim would drop dead in the shop, this would be possible.
As long as you can get thallium, etc, thin enough to go through a tattoo needle, you should be good.

The only things I know about tattooing I've seen on InkMaster, but I don't know that it would have to be a very big tattoo. When they make a tattoo a dark/solid colour, they go over one spot, time after time, building up the colour. And, adding whatever poison you want to include.

Thallium nitrate is readily soluble in water

Heck, about forty years ago, a friend of my sister's got hepatitis from a dirty tattoo needle and nearly died. Doesn't even need to be sinister, just careless.

Jeff

And the confusion of victim mentioning his tattoo was new would send the authorities looking for hepatitis or similar, rather than thinking of rare poisons.
Also, check with at tattooist, but I'll bet they change their needles frequently, so the used one could easily be hidden or disposed of. (Judging by my insulin needles, going in and out of the skin is wearing on metal.)