Methylene blue was widely used in malaria treatment, and after its relative safety was figured out, ease of access meant doctors went nuts trying it out on all sorts of conditions. And it worked as a treatment for all sorts of things, and was issued to soldiers in the field through wwii. It's a MAOI and shows up in nootropic stacks and discussions all the time.

Makes you pee blue, can treat effects of carbon monoxide poisoning, etc. There were lots of opportunities for someone with blue skin to have encountered the stuff.