Is there some underlying problem that would require the hull to be painted?
There are underlying problems that would call for a paint job. That doesn't mean a new paint job indicates an underlying problem.
If the boat suffered damage such as a collision, the glass could be repaired and the hull and deck painted. It could, most likely, be good as new. The hull could suffer blistering, as you suggested, and been repaired. It could have been repaired even better than new or it is just a cosmetic cover-up. There could also have been a dulling of the gelcoat and a desire to renew the finish. I have now had the chance to repaint my new boat but I will because it was built in '73, the finish is dull and I want to change the color from white to blue. There are 2 part epoxy paints that have harder, better wearing finish than gelcoat, they are easier to work with too. That could be the reason for the paint job.
Ask the seller your question directly. The fact that it was both hull and deck that were repainted would indicate it was a color choice or maybe a repair of a collision that damaged both areas. If there was blistering, I would expect that to only be limited to one area. However, that isn't necessarily true. Look on the inside of the hull and underside of the deck, pay special attention to chain plates and backer plates, tap it with a plastic screw driver handle, listen for weak or hollow areas. Look everywhere you can see bare fiberglass.
If it had blistering in one area, it might have it in another.
- Will (Dragonfly)