To be honest here, I'm mainly just playing devil's advocate mainly to look at it from multiple perspectives here. But I'm debating this point because I find it a fascinating topic.
Orsino was close friends with a blood mage, he traded correspondence with it, studied his work and was capable of performing a ridiculously complex blood magic ritual.
The first three prove that he had been involved in the research of blood magic for years and the last one strongly suggests that he was a practicioner himself.
Because the idea that he managed to perfectly meld his flesh with that of several other dead mages while using his soul as a stand in for a Fade spirit to create a perfectly functional living tank without having ever even tried the simplest of blood magic is kind of nonsensical.
It is nonsensical, but there is still no evidence that he was a
practitioner of blood magic before that point. Let me state that I have no doubt he studied blood magic, merely question whether he could practice it. And let me elaborate on that.
It has been stated that he couldn't have performed that ritual unless he practiced blood magic, so let me ask this, mainly as a mental exercise. When could he have found the time to practice away from Meredith and the templars? In Act 3 Orsino tells a pro-mage Hawke that if he investigated Thrask's rebellion (and he doesn't know it's a rebellion and fears they're practicing blood magic) Meredith would use it as evidence of his involvement regardless of the truth. I say we can gather from this, as well as Meredith's nature as a powerful and paranoid person, whose faults have been exacerbated by the idol, that she would've been watching Orsino like a Hawke for ANY justification to put the Circle to the torch, and she really wanted to do this. Kerras in Act 3 (if he lives past act 1) outright says that Meredith is going over Elthina's head and requesting permission for the Right of Annulment after Elthina denied her. She was going out of her way to look for evidence and had her eye on Orsino.
When would he have been able to get away from the Circle and Meredith's watchful eye to practice?
Because if he, the First Enchanter, was able to find the time to actively practice blood magic and Meredith didn't know despite living in the same place, having all the power over the mages lives and having her office literally just a few steps across the hall from his, then that ultimately means Meredith was NOT doing her own job.
To make the assertion that Orsino practiced blood magic in such an environment is almost exactly like saying that Meredith and the templars couldn't be bothered to do their actual jobs.
The same reason he is turning into an Harvester in the first place. To hurt Meredith. Orsino is a spiteful man.
What do you think would negativelly affect Meredith more? Saying "You were right all along, I am a blood mage" or "If you hadn't done anything, I would have never tried blood magic. You did this, not me."
Fair point. But it still makes absolutely no sense for a pro-mage Hawke who just fought off a horde of templars and Meredith isn't even there to lash out against.
It is contradictory. Either he was possessed by a spirit or he botched the ritual because he didn't have enough experience.
Not both at the same time.
Why can't both be true? It's already established using blood magic increases risk of possession because demons are drawn to power, the veil in Kirkwall is paper thin and is probably tearing during the Annulment due to the violence and all the magic being cast. If he lacked experience and tried casting a powerful blood magic ritual and botched, why wouldn't a demon be able to take advantage of that very moment and possess him?
Until you explain why both are exclusive options, and that a demon couldn't have possessed him in a moment of weakness while using blood magic, then it is not contradictory.
In those cases, there were obvious external signs that the mage was being possessed: brigh ligths, fire, levitation and then an horribly disfigured body.
There is no way Orsino could have been possessed without even flinching. If mages could be possessed at any time without people are in the room with them noticing, then they would be even more dangerous than they already are.
Uldred, Merethari, the templars who were possessed....no flashing lights, ugly warts and disfigurations there.
You presented the hypothesis that Orsino hadn't intended to be part of the Harvester. If this was true, then as the corpses began to circle him, he should ahev displayed some sort of distress when he realized the spell would include him.
Since he didn't, that means he wanted to be part of the monster which means he achieved the results he wanted which means he had experience with blood magic.
That or he was driven past the point of caring, which is a very real possibility considering every single one of the mages under his command, including the children, were all being slaughtered for a crime none of them had committed and most of them were already dead.