What actually causes the blister?

As moisture enters the hull it reacts with some of the components forming the laminate. This produces small regions containing a mix of highly concentrated chemicals. This concentration of chemicals is stronger than anything on the outside of the hull i.e. the seawater. This leads that cell to set up a powerful osmosis force that pulls more water into the hull from the outside in an attempt to lower the concentration. This starts to form pressure within the hull as more water is pulled in than can be contained without some expansion taking place. The result is blistering. Very often if you burst a blister the contents will spurt some distance indicating the high pressures involved.