"We are not allowed to take a closer look or hold them before we adopt "
Wow! I disagree with this wholeheartedly. Shame on that herp group.
Blisters can be caused by many things. Usually, if on the belly, they are the result of an overly damp substrate. On the back however, one might think that a rodent bite is the most likely cause.
Whichever the cause, the end result is an open window through the skin for bacteria, fungi, and other pathogens. My first recommendation is to take this snake to your herp vet. This will see your snake the best chance of survival-- systemic antibiotics and/or other medications and treatments may prove benificial after a proper examination.
Barring a vet visit, or perhaps just until you can get an appointment, you need to provide this snake with optimal husbandry. Use a paper towel substrate until the wound is healed to help keep things clean (as opposed to a particulate substrate). If you have access to hibitane cleanser, use this to clean the wound. Run the wound under luke warm water for 5-10 minutes twice daily. Some people use diluted betadine (tea coloured) for part of the flushing... that's okay too, with the goal of reducing the presence of bacteria. After drying, you can add polysporin if you'd like.
Snakes that appear thin are usually dehydrated more than anything. They probably won't want to eat, and if forced, this could cause more problems then good. If dehydration is to the extent of being visible, veterinary help is pretty much essential. If only slightly dehydrated, proper husbandry will fix it. If the snake really is only thin because it hasn't had food, it should eat if it is not too stressed out. If it doesn't eat, reconsider that it might be ill.
I hope this helps, good luck with your snake,
Ian