Here is a slight possibility. Are you letting your Chameleon spend anytime outside getting natural sun? If so, are you providing any kind of multi-vitamin supplements with D3 also? Because if you are there's a possibility that it's due to having too much D3. (If anyone asks, where I got this. I heard it on the Chameleon Academy pod cast just the other day. My Cham had the same kind of blister on his Casque) So it's possible he was getting too much D3.