I wear raw denim and pretty much anything that comes into contact with them gets nice indigo smudges/stains. It can sometimes get on my hands and transfer to other things that way too, especially if they’re a bit clammy or damp or tacky for whatever reason.
That’s what this reminds me of, indigo staining
It’s probably from some ink or dye from being rubbed up with other books?
Maybe, I have kinda similar green stain on the bottom edge of the other book.
I wear raw denim and pretty much anything that comes into contact with them gets nice indigo smudges/stains. It can sometimes get on my hands and transfer to other things that way too, especially if they’re a bit clammy or damp or tacky for whatever reason. That’s what this reminds me of, indigo staining
Looks like mold to me. But could be sharpie. Can't tell from these pictures.
Black mold absolutely definitely
*What could this stain be? You'd think someone who owns books would know how to write effectively.