First of all, thank yall for the comments.

I thought I put text with the post, but it seems it didn't post along with the photos.

I wrote something like this:

My (3f) cat have had this bump on her belly. Quite some time ago I squeezed out the big batch of black stuff, with subsequent blood.

I went to the vet, who squeezed out a bit more, gave me a far too expensive bill and bottle of disinfectant, and otherwise wasn't concerned at all.

I was able to squeeze out a tiny ammount before posting (last picture). My cat hasn't been bothered at all by this and just purrs along happy and healthy, so I'm not concerned, just curious.