This post will be rather lengthy, so feel free to skip. This is a discussion of Necromancy that leads into Dorian's views on blood magic.
This is another one of those problems where gameplay elements are conflicting with story elements. So let's take the three Necromancy spells individually and see how they changed over the course of three games...
Horror
* In DAO this was in the Sleep tree in the Entropy school: "The caster forces a target to cower in fear, unable to move, unless it passes a mental resistance check."
* In DA2 this was in the simplified Entropy tree: "Terrifying visions rend an enemy's mind, leaving the target stunned."
* In DAI it is part of the Necromancy specialization: "You unleash spirits of fear that terrify all enemies within the area."
We can see that moving into DAI they decided to shift from the (relatively simple) act of terrorizing someone to using spirits to do that work for the mage.
Walking Bomb
* In DAO this spell was in the Death tree in the Spirit school: "The caster magically injects a target with corrosive poison that inflicts continual spirit damage. If the target dies while the effect is still active, it explodes, damaging all targets nearby."
* In DA2 it was part of the simplified Spirit tree: "The mage curses an enemy with an effect that turns the victim's own body into a weapon. If the enemy dies while the spell is still active, it explodes, harming all other foes nearby."
* In DAI it is part of the Necromancy specialization: "You curse an enemy, inflicting ongoing spirit damage, and then trigger the curse in a devastating explosion."
Here the move from poison to the vague "effect" and finally to a curse (which is still somewhat vague) is interesting. I much prefer the DAO method because we know what it causing the damage. The fact that the target explodes is probably why they didn't use the spirit element in DAI as that might leave a question about what happens to the spirit after the explosion, in addition to Spirit Mark already having the component of an "attacking spirit."
Spirit Mark
* There are three spells in DAO that cause an enemy to become an ally. The first is Animate Dead in the Death tree of the Spirit school: "The caster summons a skeleton minion from the corpse of a fallen enemy to fight alongside the party for a short time...;" the second is Walking Nightmare in the Sleep tree of the Entropy school: "Hostile targets are trapped in a waking nightmare unless they pass a mental resistance check. They are randomly stunned, attack other enemies, or become the caster's ally for the duration of the effect;" and the third is Blood Control in the Blood specialization tree: "The blood mage forcibly controls the target’s blood, making the target an ally of the caster unless it passes a mental resistance check. If the target resists, it still takes great damage from the manipulation of its blood."
* In DA2 it was part of the simplified Blood specialization tree: "The mage enslaves a target, forcing it to fight alongside the party for a short time, after which the victim dies unless it is particularly powerful."
* In DAI it is part of the Necromancy specialization: "You mark a target with an attacking spirit, inflicting ongoing damage. If the target dies while marked, the spirit mimics the victims body briefly to fight on your behalf."
We can see that they really refined the ability as they moved through each game. It appears they combined elements from the DAO spells for use in DA2, and then just converted it, slightly altered, for use in DAI. This is the only spell from the Necromancy spec that is related to Blood magic.
It's a shame that the Entropy school isn't in DAI as I feel that would be a great fit for Dorian. Indeed, I'm sure he knows these spells although they aren't presented in the game (like healing is not). It would be interesting (and scary) to see him using his full arsenal.
Necromancy is not Blood magic. Blood magic was supposedly first taught by the Old God Dumat, and Blood mages normally learn it from a demon. One of the ways you access the spec in DAO is by making a deal with the demon possessing Connor. There are elements from the Spirit school, the Entropy school, and yes also the Blood school that made it into the DAI Necromancy, so I don't really feel that there was hand waving as far as this specialization is concerned.
"Spirit binding" is basically what Necromancy involves, yes, which is one reason Solas does not deride you when you say to him that you chose this spec to learn more about the spirit world. Cole elaborates further by explaining that you aren't manipulating complete spirits, but only minor bits. Essentially, a necromancer temporarily summons a spirit into a corpse. This is done via a connection to the fade that all mages have and blood magic is not required.
The codex entries are somewhat limited when it comes to this since both of the ones involving Necromancy and the Mortalitasi are written from a personal perspective and don't contain a lot of information. The best source is actually Viuus Anaxas, the mage trainer [youtube link].
That said, I'm sure there are mages, like Anders, that might feel that Necromancy pushes the boundaries. But as we know, the distinctions are somewhat more refined in Tevinter. Dorian perfectly illustrates the problem with Blood magic: "Blood magic isn't inherently dangerous. Using your own blood or that of a willing participant? What's the harm? The problem is that what's permitted only gets you so much power. And what if you need more? You always need more. That's where we get into sacrifices and demon-summoning. None of that is done -- not officially. Behind closed doors, it's a different story. Real blood magic can give you an edge, a leg up against your opponents. It's safe to assume that any mage of rank does it. The rest are quietly shut out of power, to put it bluntly."
When talking about blood magic at the PAX East panel in 2012 David Gaider said that the fear of blood magic "is based on people who have that kind of power, the ability to influence other people's minds, the temptation to misuse it is a corruption. That kind of power is corrupting, right? That's the danger, not a physical 'I suddenly turn to the dark side and my eyes have gone black' kind of corruption."
Note the word "temptation" here, and recall the word on Dorian's tombstone in the fade. Talking of power Dorian says "And what if you need more? You always need more." And that is where the problem comes in.
I liken Tevinter's use of blood magic to the use of steroids in sports. You think these guys want to be considered to have cheated, have their testicles shrink and other adverse health effects, or suffer from "'roid rage"? Of course not. It's a Catch-22. They "have" to use it to stay competitive with those that do. So everyone is using it because everyone is using it.
I found Dorian's comments about it to be interesting and would have liked to ask him (ignoring the recent issues...) how that relates to his father's views and his own by proxy. Halward was ready to resort to blood magic out of desperation, not out of a desire for power, and he taught Dorian that "blood magic is the resort of a weak mind." Halward had a moment of weakness born from desperation. But what I wish to know is, if Halward was against using blood magic, how has he stayed competitive with his fellow Magisters? Or does he not care about that? If he wanted a "son who could become archon" he should have cared and part of Dorian's teachings should have included the "anything to get an edge" mentality. But we know that isn't the case. I don't really feel that this jives with the Pavus family ambition we are presented with, and actually shows Halward in a better light than one might otherwise see after learning of the whole incident.
What Halward did, and planned to do, was terrible. But I also consider the type of man that raised Dorian to be the man he is. Despite his vanity and arrogance, Dorian has a high level of morality, goodness, and patriotism; that didn't come from nothing. A significant component of the betrayal, in addition to the "change" aspect, was also that Dorian's admiration and respect for his father was destroyed when Halward went against everything he had taught Dorian.