My complaint is not "ooh, blood magic" it was that the plan was so ludicrously STUPID. Sure they were afraid, but they didn't even bother to find out if Wardens all over Thedas were hearing the Calling. They just assumed that if the ones in the South all died THE WORLD WOULD END IN A BLIGHT. The entire plan was ridiculously stupid, and Clarel should have known better. Who thinks, "I know, let's kill most of our members and bind demons and go hunt down archdemons because some random Tevinter magister thinks it's a good idea"? Clarel, obviously. While I am loathe to agree with an arsehole like Livius Erimond, he was right when he called her a stupid ******.
I can get spur of the moment stupid decisions because you're afraid, but this was a plan that had been thought out and executed for months. It's not like the momentary panic that seizes you and makes you do dumb stuff (that happens to everyone). Those moments when you just have to say after, "Crap, I don't know why I did that, I wasn't able to think clearly." That makes sense. "I was scared, so I took the advice of a complete arsehole to murder half of my order and raise a demon army, and, oh, yeah, it took months of planning" doesn't make as much sense. It's stupidity, born of hubris, though started in fear.
Then again, there's a lot of "How can you be so freaking STUPID?!" going on in DA:I. Templars were being led by a demon (uh, wut?) as were the Seekers, the rebel mages had zero plan apart from "we rebel! yay, now we're free!", the Wardens, the Chantry with their "let's ignore the edicts of the previous Divine and denounce the one person who has any chance of saving the world", pretty much all of them. I don't know that I'd say Gaspard/Briala/Celene are necessarily stupid, but they are pig-headed, arrogant, and all in need of having their heads cracked together.