This will sound weird, but whether blood magic or not being justifiable is also dependent on the type of magic. According to the Chantry and the templars, anything involving blood is considered blood magic. Some groups even outright ban studying human anatomy because they fear blood magic so much.
But the Grey Warden joining is a form of blood magic with its use of darkspawn and archdemon blood. The phylacteries templars use are also a form of blood magic. Merrill says that the spell she used to cleanse the eluvian shard didn't even require blood magic, but she lacked the power herself, Merethari refused to help her, and she lacked the lyrium to amplify her spell, so blood magic was her last option.
I think it doesn't matter if a mage throws a fireball in your face or boil your blood from within. Both would be excruciatingly painful ways to die, yet one is vilified and the other isn't based entirely on the school of magic. However, there is a huge difference giving yourself some amplified rock armor and controlling the thoughts of others and force them to make decisions they don't want to and essentially take their free will.
In Inquisition, I predict we'll be seeing four types of blood mages. Those who are "mwahahaha, I'm insane and evil!" Those who are researchers and want to study magic, but because the Circle's declared their indpendence they find themselves lacking lyrium, and only want to amplify their own spells because they lack the lyrium otherwise required. Those who are forced into desperate situations and it's a do-or-die scenario. And finally, those who fall into the category of the two I previously mentioned and then became corrupted by the power, addicted to it, and look for any means to justify it continually.
In cases where it's do-or-die, I have no problem with it. In cases where it's research, I want to take it case-by-case, is it your blood your using and no one elses and what you are researching. Nearly every other case, I don't see much justification. And those who want to use it for research purposes, I'd want them under constant observation by those with templar like abilities, and they agree to this watch before they learn blood magic to begin with.