Others more qualified than I will surely chime in here, but for what its worth,
I tend to agree that most cases of blistering in single-skin hulls are largely cosmetic and do not threaten hull integrity (I'd be much more leery of deck delamination than average blisters - but naturally blister-free is better) In the case of cored hulls, it's a much greater concern due to the relative thinness of the skins.
I'd listen to your surveyor rather than the seller with regard to issues like this.
As to Blister repair jobs having been done, it depends on the methods used and the skills and knowledge of those who used them. There are a lot of practitioners of this, each with their own ideas and techniques and they can only be evaluated over time.
Here on the west coast we have had rashes of "Leaky Condos" (not floating ones), and when buying a condo you have to decide to either buy one that's been fixed and hope it was done right, or buy one that has never been fixed and hope it won't need to be..... It's kind of a similar situation wrt hull blisters and most boats in the 70s-80s vintage.