From Geldauran's Claim, they sound more like rebels against the established order. It IS possible that they are responsible for the Blight, but being demonized for for their rebellion, it seem entirely possible they have been confused with some actually demons in Dalish lore. We don't know yet. Not enough information available. However, this is yet another thread of the Blight going back before "the first Blight".
I'm going to preface this with the fact that I haven't slept so I may be a bit more loopy than usual. That being said, I don't have a problem with the idea that the Blight existed before the rising of the first archdemon. Cory says as much. I just don't believe that it's the first cause. Someone or something was behind it. And if this was the real world, I'd be willing to accept the premise it evolved, but Thedas doesn't completely function by the same rules. I can't believe it's just random natural selection, especially coming out of the Fade.
The Blight is pestilent corruption. That initial corruption has to come from somewhere and that might very well be demonic corruption. It would certainly explain the similarity between demonic and tainted environmental effects. Just because demons can more easily be corrupted than blood or lyrium, does not necessarily indicate that they are in fact different. Afterall, corrupting some blood is very easy, but corrupting some lyrium is very, very hard. And yet they both have the same Blight.
Here's the thing. We've seen demons revert, Solas quest is one example. It's been implied that during Asunder that Cole was corrupted and found his way back. The fact he can move more fully back to a spirit during his own quest, should you choose, confirms there's a way to recover. And then there's the fact that the Seekers have that resistance. If it comes from the vigil, which is the only thing that makes sense, then something about spirits may be resistant to the Blight. I don't have a problem believing there's a connection, Anders does point out that if Bartrand wasn't a dwarf Anders would have sworn he was possessed, but I'm pretty sure that demons aren't Blighted. Spirits aren't really alive in the way mortals are.
This spirit resistace also may be confirmed by Leliana. IHW a journal points out she's got a resistance to the Blight. My surmise is that might have come from dear old Dorotea. The two memories the IQ got back at Adamant really don't give too much, like what was Cory taking from the Divine? How did the IQ and the Divine survive the explosion and end up in the Fade? Why didn't the Divine need to die to activate the orb? Dorian also points out, if you take him, that the spirit the IQ follows doesn't act like a spirit. It could be her soul, but why would a mortal soul have such power over a demon like the Nightmare?
To everyone in Thedas, Red Lyrium was just another kind of Lyrium (on crack, sure, but still a different kind of Lyrium), the fact that Solas just KNOWS it is corrupted says enough.
He may know, but saying it was magic that corrupted it is really vague, and at the same time makes sense to everybody who was there. We don't know exactly when he went sleepy, so it's possible he may not know everything. And if he did know, why give his orb to a Blight corrupted magister? That's the disconnect for me. I can't see the Dread Wolf taking that kind of risk if he knew the full extent of what was going on.
There was a dragon in the thaig, there was also a dragon in that underground temple where the werewolves were gathered in DAO.
Solas isn't thrilled about killing dragons, and Yevena was watching over a whole herd of them. And of course Mythal's connection to them.
Then there's who called to Corypheus and friends... What if the old gods were already asleep, and someone else wanted to set the Blight on Thedas? Destabilize the largest mortal power at the time by corrupting it, and set loose something that will keep returning and possibly causing more breakdowns, keeping anyone from reaching heights similar to the ancient elves.
Then there are the dragon cults. They get a mention in DAO at the Temple. Some of the points made were things like how do the cultists live among the dragonlings, how does a high dragon know that the cultists will protect her young, and how did the cultists know to drink dragon blood for its power. Too many threads not enough information. I hate that.
I think the next game will really explore all of this and I hope descent will give us a few more clues.
We can hope, or I'm going the Edmonton and strangling people until they talk. I HATE not knowing things.