The blistering could be caused by a pocket of air inside the silver clay, say for instance if the clay had been folded over itself, trapping air, during the shaping process. When heated the air expands and needs to be able to escape - this isn't a problem whilst firing silver clay; as the air expands during heating it can escape through the still porous clay surface and, once the surface becomes hard metal the air won't expand any further (until heated again!).
When you solder "traditional" silver with air trapped (say a bead made out of two domed discs for instance) you would make a small hole for the expanding air to escape, or the piece can blister or explode (as you've just noticed!).
I could also be caused by trapped moisture. If the silver clay has been quenched after firing, or even just washed, this can sometimes happen. In particular if it has been quenched as this seems to almost "push" water into the silver.
If there is a risk there is trapped water you can avoid blistering by ensuring it is fully dried out, for instance by heating the silver slowly, using a big bushy flame, moving it over the silver from a distance, slowly getting closer, letting it gently warm up - before you solder.
With the pieces that have already blistered, you could try gently hammering the blister flat (has worked for me in the past, I did the same when I first started out!) with a nylon hammer. Put a small piece of untextured fabric or rubber between the metal and hammer to avoid marking it.