Blisters occur in the strand-mat layer which is a cosmetic layer above the cloth layer of fiberglass. I would only be concerned with blistering if the below waterline hull is balsa or open-cell foam cored. Having said this; I did do a blister job on my boat when I purchased it; at the time thinking preventative maintenance of them was the proper thing to do.
After getting the hull stripped down to gelcoat I found even more blisters that were not visible beneath the paint; as well as many poorly repaired blisters. Fixing all of them was a chore. I did it following the procedures given by MAAS epoxies. Go to their website; learn how to mix epoxy with fillers and do it yourself if you want it to be done right. All you need for tools is an angle grinder to remove the blister and a good sander after you fill them.
If you sand down to gelcoat I also recommend painting the entire hull with several layers of epoxy to prevent blistering in the future. My boat was in the water for 6 years after barrier coating with 5 coats of epoxy, 1 coat of Pettit barrier coat to tie to the bottom paint. No blisters after hauling again.
I found some blisters on the rudder; but I think the surveyor was a little too aggressive with the hammer because you could see a visible pattern of them. Which makes me wonder if hull blisters on many boats could be a result of one or multiple hull surveys