I'm afraid if your boyfriend had herpes 6 years ago then he still does.... its a virus that once you have it, it doesnt go away - even if the blisters or lesions do. He is less likely to spread it if he doesnt have active ulcers but it is still transmissable. There currently isnt a "cure" as such, only treatments.
The most common cause of blisters in that region is herpes unfortunately, and the blisters can occur anywhere in the region - internally within the vagina and externally around the vulva, perineum and anus.. The other things it could be are:
a dermatitis or allergic type reaction to the creams or yeast treatments you've been using,
or a staph aureus skin infection (these are usually painless) but don't look like "blisters" as such I'm afraid.
What did your gyno/doctor say? ususally a gyno can spot herpetic lesions straight away.
Good luck, let us know how you get on....
PS you should definitely use condoms till all is sorted out! (and make sure your partner has STI check too) I always recommend that people (even those who are strictly monogamous have regular STI checks because you just never know :) )
ik its been a while but did you figure out what it was?