So, in DA:I - how would Blood Magic stack up to constantly having to fight demons? Or that whole section in the fade? If you'd been speccing for all blood magic, by all rights, you'd be useless there.
I can see 2 solutions.
1) The Anchor/mark.
2) You are still using normal spells.
Besides, *You* Don't pick your trainers, Josephine does, she sends word out. Leliana finds your Rogue trainers, and Cullen finds your warrior trainers. You've no freedom outside of picking from the 3 that showed up.
As said, I see no problems but a small dialog line for adding a Blood Mage trainer. If not, you could just add a side quest somewhere.
And I doubt Cullen would follow - this is a man who was tortured by Blood magic in DA:O, a man who saw first hand how much Blood Magic was being used in Kirkwall - if anything, It'd be like the Sten vs Warden fight in DA:O.
He'd follow out of neccesity - as any solider do. Besides, the army follow him -- not you. Some snarky comments from him, or having him and Cassandra constantly checking up on you. Blood magic does not fry your mind. It simply adds a chance of problems, which can be countered with some anchor-magic-stuff.
The go to argument against Blood magic seem to be "Innocent dies!". However, Blood Magic does not require innocent blood -- it requires blood. I, for one, have got enough blood to fuel 5 blood mages from Hinterlands alone.
Companions in this game *Can* and *Will* Leave if they don't agree with you, just like in DA:O, where you wound up with Dog, and Alistar/Loghain if you were an Puppy-eating monster.
You also no not need any of them to suceed. The only people required are the 3 non-companions. All you need is an army. I have no need of Iron Bull, Sera, Varric, Cole, etcc. The only person I will consider vital are Cassandra and Solas. Solas would not leave, whereas Cassandra would probably not leave either. She feels a failure for the death of the Divine. She'd do anything to fix this. You're her solutions, no matter if you draw power from ripping the heart of a living enemy (Reaver), stabbing them in the back from the shadows (Assassin), stealing life energy from the dead (Necromancer) or by empowering spells via blood (Blood Magic).
It would require more work to fit it into the game, obviously, which is why it isn't in the game. That, and the obvious no-healing-deal which makes it require more work to implement.