I recently went to an exhibition on mushrooms at the botanical garden. Coprinus comatus was the main attraction, as it grows locally and is "mushroom of the year", apparently. If you harvest the caps and let them sit for a couple of days, they liquefy into a brown sludge. Strain that, and you have a beautiful brownish-black particle ink, with the particles being its spores. When young, they are also edible, and they stay non-toxic when liquid. Put a couple of drops of an antibacterial oil in it, and it keeps for some time. (Do not cook to sterilize though, as it congeals. Ruined the first batch like that!)
wait, if the pigment is the spores and you put anti-bacterial in (not a fungicide), your drawing will grow mushrooms, when the spores growth conditions are met (e.g. some moisture gets in)
Oh oh! OP, try this and see if it works! Then you can sell your drawings as mushroom starters!
I will try that with the second batch, that's an awesome idea!!!