One is new skin that was nearly ready to appear after the next shed. The scales have a normal pattern and of proper size. I can't say exactly what happens to cause that. The only fluid-filled blisters I've seen are filled with blood tinted fluid. It sounds to me like the parting skin got a very hot burn, but he has new protective skin underneath, to protect him from an infection.
The other is when you can see the smooth raw red meat under the flap. sometimes the flesh is tinted yellow or some other color. This is the color of the bacteria colony. I've seen it. It's caused by a severe burn. If the raw flesh is also flawed, then it's a third degree burn that must be treated right away.
If you get him to a vet soon, he'll survive. If he doesn't get vet care, he will certainly die of sepsis after you spend $5,000 trying to save him when it was too late.
It looks to me that your ig's back was too close to a concentrated heat source, as if he leaned slightly into a light bulb. Vets trim the extra skin away and prescribe special antibiotic cream plus an antibiotic injection. My ig (when I was a beginner) had a belly burn from his front legs to about an inch from his vent. The blister was red-tinted clear fluid. The fluid dried out and he grew a sturdy scab over the whole injury. I could lift a corner of the scab and see the infection that had taken hold. I figured it was a fatal injury, but the vet said he'd seen it before and that many survive it. my kids had named him Iggy. He need a n entire year to heal completely, and he never got sepsis, and the new belly scar was made of tiny disorganized white scales. Then he lived a pretty long life, considering how little I knew at first.