Its caused by a bacteria commonly found in the wild and its very contagious. Good thing you seperated it, but quarantine procedures would have likely allerted you to the problem a little earlier. Wild caught snakes are usually more stressed, and used to a wider range of temps that are commonly kept in captivity. In such a rainy year its common for wild snakes to be found with blister, and they are usually the easiest to find because they need HOT HOT temps to fight off the infection so they bask extra long under whatever you found it.
There are a few more tricks to get your snake into full recovery mode. The first I use is extra thick paper towel substrate, you will know if its gets wet or soiled at all. The hot end of the cage has to be at least 100F, the cool end the normal 78. NO WATER should be present!! I cant emphasize this enough, they will search out water which only exacerbates the infection. They will be seriously dehydrated as the water that creates the blisters comes straight from the immune system so hand watering twice a day will be in order. Once under control it will go into a rapid succession of shed cycles, usually 2-3 without feeding. The dehydration and malnurishment usually kills the animal, but lets assume its survived to this point. Now the blisters will have fused the old skin onto the new so shedding is impossible. DO NOT SOAK, the usual remedy for a bad shed. You will have to hand peel off the old skin, this will cause legions that may be permanent. Right after a shed and every day use swabs and dip them in hydrogen peroxide and disinfect the snakes as needed. Its ALWAYS important to remember how communicable this disease is, wash hands and instruments before and after handling the snake and best to bleach the container it was held before and during this process. Good luck!