Black Smothering Instructions Cat Smothered/black Stuff?

Cat smothered/black stuff? - black smothering instructions

Earlier today, my cat was almost suffocated with a plastic bag. Luckily, my mother came home from work on time, and remarked in her room on her bed. He was foaming at the mouth, and there was black stuff in the plastic bag. Out of curiosity, what was the black stuff? It's ok, but I'm a little bit about what he was worried.