Yeah, he could have made that more clear.
Also, it makes the Templar ending even less justifiable.
Orsino was a blood mage. The most disgusting kind at that, as he supported what Quentin did. And he was a leader of this Circle. Apart from that, almost everyone we meet is a Blood Mage. The thing is, that Blood Magic is hard to learn. It's not just knowledge that's laying around. That means, that there are people inside Circle, who are actively teaching Blood Magic. Orsino was leaking forbidden knowledge to an apostate - he could have just as easily allowed Circle mages to have access to knowledge about Blood Magic and Necromancy.
Sure, not every mage there was a Blood Mage - but there must have been some, high in the Circle hierarchy, who actively taught Blood Magic.
I'm a little fuzzy on the lore on this one. If you learn blood magic from a book, or from another mage, does it intrinsically make you more prone to demon possession? If it does then abominations must be a serious problem in Tevinter yet I can't recall the lore saying much about that. I thought the demon possession problem was only an issue when you needed to deal with a demon in order to learn blood magic.
Blood Magic makes it easier to tear the Veil - which in theory makes possession more easy. However, the mage still has to accept demonic possession - rember Uldred in Origins, who asked mages a question before they became abominations? "Do you accept, the gifts that I offer?".
I doubt that demonic possession is a large problem in Tevinter. Magisters are obviously among the most powerful mages in Thedas. Becoming an abomination means losing control, and basically almost dying. There is not reason why a high ranked Magister would become an abomination. As to lower mages, they are supervised by experienced masters. There is no source of distress or danger (in a form of Templars wanting to execute you), that would cause mages to to give into demonic possession willingly. And inexperienced mages are supervised in the Circles.
Anders is actually at least partially right. Becoming abomination is caused mostly by Templars. Imagine a mage, escaping from the Circle, because he doesn't want to spend his whole life in prison. If Templars find him, he has only three options:
1) Be executed by them
2) Become Tranquil
3) Turn into an abomination (giving away the control into demon) and try to fight with new power.
For many, the third option is preferable to dying or becoming Tranquil.
In Tevinter, that is not a problem. Mages have power and wealth. They are feared and/or respected. They have no reason to turn into abominations willingly, as it's an act of desperation.
There is one issue I have with DA in terms of gameplay. Abominations are too easy in combat. They barely use any spells and attack primarily in melee. A powerful desire/pride abomination is able to kill an entire squad of Templars with ease. That is not reflected in gameplay.