the swollen scales are water that has moved underneath the unshedded skin. A snake that soaks for 12 hrs might get a little water logged- especially if he's shedding. Blister disease is usually accompanied by redness. A lot like when a boa is kept in damp unclean conditions and the belly turns red(septicima, fungus). I've only had a blister disease problem with a leucistic ratsnake that had a genetic predisposition for skin problems. Blister disease is a fungal infection, which a vet might perscribe pipercillin- but like any fungus atheletes foot spray will take care of it. The more liquid types not the sprays that produce a powder residual. I can't think of the name.. all I can think about is "tough actin' tinactin" but I'm pretty sure that tinactin spray turns into a powder. Maybe like lotrimin? Whatever you do, do it PDQ. Fungal infections can help bacterial/viral infections take advantage of the situation and make MORE problems.. ie menengitis.
Good luck-
Matt Schubarth
Pet Nebula
2100 Stephens # 116
Missoula, Montana 59801
406 541 9929