Hi This is problem is Osmosis.
To cure the problem we can do 2 things:
1. Use heat and moving air to pull the moisture back out from under the gelcoat. In the workshop here we use a hot lamp and fan heater. The part will need to be left for several days for the bubbles to retreat. This will use a lot of KW. Or you could put in warm dry place such as an Airing Cupboard or boiler cupboard for a couple of weeks.
2. Pop the bubbles, dry the area out and reapply gelcoat. Personally I lean towards the 1st option.
To prevent the problem from reappearing:
Only use the padded rudder bag for transportation purposes and make sure the blade and bag are dry before use.