Oh ye I understand your points, as for the voices and mind control - the former would be just scripts added to randomly play some creepy stuff when your out of combat or reach certain point in the story. About the mind control - what i was thinking was an increasing chance to loose control of character, the more blood magic spells are cast during a single encounter, lets say at the start its 0% after 3 casts it becomes 5% then every next cast increases by another 5%. When you loose control basically makes your PC one more enemy for companions to kill. It's like in DAO when you basically died just here u also become enemy. Of course it could be stacking debuff numbers could be tweaked or some other kind of penalty - could be even linearly tied to the story as in your corruption progresses with campaign and you have to deal with the demon at some point - you know like most linear rpgs do.
As for consequence - look at merril, everybody in the party knew what she was up to but they just dealt with it until you had to do some quests about it , could be same here for PC. Those consequences could loose you some of the companions, or not let you recruit a group for inquisition and so on. You could even not use blood magic spells at some areas for npc not to notice - the possibilities are endless.
If blood magic would bring dire repercussions that would only enrich the story and get you more immersed. Just like in DAO you couldnt recruit all groups, in ME2 your companions could die, here you could also loose them or some assets.
I get the mechanics you're proposing. But would anyone want to play a spec that randomly makes you lose control of your character? And really, the lose control part is basically when you become an abomination, right? And that's supposed to be irreversible. So, game over for your MC, no?
As for Merrill, yeah, all the party members tolerate her. But most only do so out of loyalty to Hawke. Anders, Fenris and Sebastian, for instance, have serious problems with her Blood Magic, and would absolutely turn her in if not for Hawke. There's actually a party banter between Sebatian and Fenris about how they should really turn in all those mages they've been running around with but they don't because neither wants to antagonize Hawke.
The repercussions you suggest are pretty much the kind of thing I'd expect to see. But some of those are big things that would impact the entire story, it would become an entirely different game. Just for one spec of one class. So I get why they didn't add it in this time.
That got me thinking though, if they did want to implement a spec that would make you insta-hated/feared, it would work better if they had one of those for every class. Warrior, for instance, has the Reaver spec which seems to be pretty close to what Blood Magic is for mages. If Reavers became more well known they might gain a feared status not dissimilar from Blood Mages. And for Rogue you might have a spec that represents them joining/allying with a particularly feared group of assassins, or something to that effect. That way each class would have an "evil" spec, so that content could be used for more than one spec of one class, and people could access it even if they don't play a mage.