Black, gross, probably toxic.
I took down the shower doors that have been an annoyance for me for years. I guess I should have gotten some Goof Off instead of just trying to pry the brackets off the side wall because a whole column of tiles came off too. Bad news: new project now to replace the tiles. Good news: I found mold behind the tiles which definitely needs to be removed before my mom moves back in. [Or if I reverse that – bad news: mold. good news: I get to learn how to replace tiles!] I sprayed with a bleach solution and began cleaning. After doing this a couple of times it was gone. Now I’ll have to check other moisture spots and get everything sealed well.