Sandblasting with sand, is not an acceptable way of removing bottom paint and blisters as it requires very great skill to keep the bottom of the boat fair. Generally softer cutting agents are a little better for that kind of thing. The good news about blasting is that it does not build up localized heat which can promote ongoing blister problems.
Frankly with only 6 quater sized blisters, I would suggest repairing just those blisters, in which case sanding is fine.
As to bottom paint build up, the yard should be removing multiple layers of bottom paint when they sand for the first coat of each bottom job so that the boat does not become ''paint sick''. With the high speed air driven sanders that are so common these days this should not be a problem.