What would Snape do if he found out that Umbridge was using the blood quill on Harry?
Snape isn’t stupid, and he’s used to working with evil people he can’t just sic the ministry on. (Death Eaters)
He might tell Umbridge that Hogwarts had a policy against injuring students as punishment, but if it were late in the school year, when it was clear Umbridge could do whatever she pleased, he wouldn’t.
Instead, he would probably flatter her, thanking her for teaching a student a lesson. He would convince her of his sincerity by reminding her of his dislike of Harry.
He wouldn’t tell Dumbledore. I think he would realize there was nothing Dumbledore could do. If Dumbledore had tried to intervene, Umbridge could have had him removed.
Snape would guess that Umbridge was using the blood quill on other students too. He would probably hate that.
But he might stick with the habits he’d developed spying on the Death Eaters, and stick to his strategy for dealing with all the sick stuff the Death Eaters did that wasn’t useful information, life or death, or something he had a decent chance of fixing without blowing his cover: do nothing. Spying stinks sometimes. But as an occlumens, Snape had to be skilled at putting things out of his mind, including things like that.
But if he thought he did have a decent chance of helping without “blowing his cover” with Umbridge, he would put his mind to work. Maybe he’d find out who had detention with Umbridge, and get a potion into them to reduce pain, replenish blood, or speed healing. Or maybe he’d just start teaching those students’ classes how to make those potions, and trust they all had the brains to drink some of the stuff before it got vanished at the end of class.
Or just slip potions into their dinner goblets.
What would really get interesting is if he tipped off Fred and George. He wouldn’t do that, because A, he thinks they’re nothing but trouble, and because that could end in a lot MORE blood with the way the twins operate. But it would have been fun to read how that played out.
He might, though, tip-off other teachers and plot with them how to help the students. He might decide not to tell McGonagall, who might confront Umbridge, enraged, and get herself fired. If he needed to avoid suspicion by dividing his secret healing duties between Sprout, Flitwick, and himself, he might have a private conversation with them.