is it the actual skin of the rock or the paint itself or both??
First of all you need a fan or suctin vent or a heater to rid of this moisture. If you do not have one, this will never ease for you.
OK, if the water has gotten under the paint and only the paint has bubbled you can sand this down and feather it out and spackle it and then re paint. If this has bubbled the skin of the rock, in my opinion the rock has been compromised and nothing will stick.
There are some good quality bath and kitchen paints for just this situation found at Paint Dealers. Sherwin for an example has a product that will fight this extra moisture and withstand.