Jowan isn't very smart but he knew enough to keep the fact that he was a blood mage to himself seeing as how he lived in a place where such a thing is punishable by death. Can't blame him for that. I wouldn't tell anybody either. What I don't understand is why there isn't an option to run off with Jowan. You finally have a chance at freedom but instead you would rather stay and leave yourself at the mercy of your Templar jailors after crapping all over their laws? Wtf?!
He's not an idiot for not actually yelling "Please kill me! I'm a blood mage!" at the Circle. He's an idiot for using blood magic in the first place because he's jealous of the Warden. Seriously, he picks the most hated school of magic - the one that's an automatic death sentence - because he's frustrated he isn't that good at magic. To quote the Avengers "Not a great plan."
As for not being able to leave with him: your phylactery was in Denerim. You even have dialogue to that effect.
I also can't blame him for trusting the famous Fereldan freedom fighter hero Loghain and agreeing to work for him. Jowan had no reason to not believe his lies. Your average Fereldan would have fell for it too. What I don't understand about this story is how Connor got possessed in the first place because if mage children can become meatsocks, why doesn't the Circle put them through the Harrowing? Why wait until they're adults? If they've survived that long then maybe there's no need to do it at all.
Loghain asked him to poison Arl Eamon. You have to be a complete moron not to see that this is 1) illegal 2) because it is murder so 3) Loghain clearly is making a power play.
As for the Harrowing, it's incredibly dangerous. Children can't even use magic properly. Putting them in front of a demon is a great way to give that demon a new meatsuit.
I don't know how long Jowan was at the Circle but it must have been a good while. Not sure how old he is but he's been walking the Fade every night for at least 18 years and hasn't gotten possessed. Maybe he's not as weak willed as people think. If the demons don't notice him because he's a weak mage then why put him through the Harrowing at all? Weak mages should get a pass. If they might notice him now that he's gained some strength from blood magic then every second counts and action should be taken immediately to negate the threat. Yet he's been dabbling in the oh so terrible forbidden magics for a few days at the very least and all they're doing is watching him just like they watch everyone else. How does anyone know that he's highly susceptible to possession anyway? Is there a spell or a test for that? If someone is found to be an inevitable Emily Rose then why not take care of the problem immediately? Why wait and put everyone at risk?
No, he's no highly susceptible because of blood magic, he's highly susceptible because he's a moron. If Loghain asking him to poison Arl Eamon looks like a great idea, and if he uses blood magic in the first place because he's jealous with absolutely no plan not to get caught... then if a demon wants he's easy picking.