Rudder blisters are not to serious, in most cases.
I had a rudder with a few blisters and also when I hauled for painting found that I had water weeping out of the rudder.
I had to sand down to bare glass to find the cause and found many cracks and some impact damage.
Part of the front of my rudder was starting to split open in very small areas
and saturating my rudder, so this was why the blisters were happening.
Sand down the rudder to bare glass, drill holes and the end of the cracks and also check for other damage.. drilling small holes and the end of the cracks will help you find out if the rudder is saturated in some areas.
go to my post about my rudder repair, although it may not be what your problem is, may help you see that tis not so scary a ordeal and what to look for damages.. The least you can do is repair the blister locally but ideally you need to find the source, a water damage rudder/saturated rudder is a very dangerous thing. think about it.. losing your rudder to broken post due to corrosion under sail, trying to get back to dock with no rudder, cost of towing, cost of hitting someone.
If your boat is on the hard, best time is to do it now. Drill holes in the cracks if any and check for water, chase the water if you find with small holes to determine how far it is or isn't.
If u have any questions you can reply back, I am more than happy to help you with anything I can.
Mostly the community is as well.
http://forums.hunter.sailboatowners.com/showthread.php?p=1120285&highlight=wesleysherman