So I noticed this happening to my Boston Terrier about 6 mths of age, maybe after her spay, but I definately noticed after her 6 mth shots.
We had similar problems, red type blisters, constant scratching and did all the tests with no conclusions. If her skin smells, she may need an antibiotic treatment. Did u look into Psoriasis, it's a condition that is smelly. Marty too spends her day looking for back scratchers....
We did it all and have resorted to steroids unfortunately. Are u in touch with the breeder by chance? If you get any answers I would appreciate your input.