Literally logged in for the first time in a decade at least to comment lol.
I've noticed with the Easytouch rigs the bags blue ink runs off on skin when wet, and if it's on your fingers when you ball up a cotton you can draw it up and tattoo yourself inadvertently so to speak.
I put this together after me and my girl both ended up with one or two blue spots on our arms, and in over a decade of IVing never had it happen before. Soot/carbon could do similar as noted above.
To add I've noticed that hitting the same spot repeatedly can leave a grayish blue scar or track mark that takes a bit of time to fade as well.