Natacha, I'm sorry that your question wasn't answered in a timely manner. We don't have many avian vets on this site. This is a case that needs a hand-on exam to better understand what you're seeing and describing as a blister and how it might best be treated. Ordinarily, I ask for a close-up photo of skin cases but I understand how difficult that would be to get of a canary. Please see if you can find an avian vet near you through this site: theparrotssocietyuk.org/veterinaryadvice/avian-vets
For now please keep him warm by shining a 100W bulb into his partially covered cage (not at night when he needs to sleep) or by taping a heating pad set on its lowest setting around the base of his cage. I would appreciate your telling me the active ingredient in the fluid, please, so I can comment further on its danger to your canary.