Blisters can come from a few sources. Inside going out, dry mold issues from when the boat was made, as well as from when it sits with scars on the bottom, and transfer in the hull.
You will find blisters more so on fresh water boats than that of salt, because salt water is denser.
Are blisters a big deal?? Not in my opinion. From all my years as a repairman I can tell you that like Wes said, the deck issues, structural, etc are a much bigger deal..... Blisters, not so much. I can tell anyone how to repair them super easy. Just takes some work.