But wasn't it her keeper that screwed everything up by being Possessed by the demon? It doesn't end in tragedy because she goes to far with it and dies but because her keeper tryed to stop her from using "evil" blood magic.
Just because it wasn't Merrill who got possessed and killed (which would have been what happened if Marethari hadn't taken the bullet for her), that doesn't change the theme of her story. It was basically an Aesop Collateral Damage plot. She learns blood magic from a pride demon who continues to manipulate her with more knowledge to get her to free him, despite the warnings by everyone around her, and she is only saved because her mentor takes the fall for her (which Merrill now has to live with the guilt of). Consider that the pride demon Merrill learned blood magic from is named Audacity (defined as, "boldness or daring, especially with confident or arrogant disregard for personal safety, conventional thought, or other restrictions"), a word which describes her character throughout her arc.
The case can be made for "ethical" use of blood magic (provided the caster abstains from the more diabolical parts of it like demon summoning/mind control/thought-invasion/sacrificing others), but Merrill herself was far from a subversion IMO. She struck me as a textbook example of how resorting to forbidden magic destroys everything you hold dear regardless of your intent or how careful you are.
If you want an actual subversion, the closest I can think of is Fallstick. He's a blood mage with a seemingly positive depiction, using it to heal and protect and suffering no consequences for its use in the story (to my knowledge, I haven't read the comics myself).
On this topic, I was always rather sad that you couldn't share experiences with Merrill if you were playing a blood mage Hawke. That was such a missed opportunity.