I believe the keel is a solid pouring of lead. I also think that there may be some epoxy gel-coat that covers a portion of the keel and that maybe what has bubbled out. Im sure the smile is the source of the water that froze. Best bet is clean out the smile, caulk, tighten keel boats and fair the smile with fiberglass. The keel stub is solid with the haul. The age of your boat more than likely has a 2X4 which was bondo-ed in between the keel stub and keel. Note in 1984 they used bondo that absorbed water. 

Hm.  From your post it seems the boat is now in the water, correct? 

We grounded our '91 a few years back in two feet of sand overnight (let me see if i have relevant pics) and discovered some interesting fiberglass resin fill jobs on the starboard side of the lead keel.  Not sure if it was a bad mould fill with the lead, but below the fiberglass portion which extends down about 8 inches from the hull, the keel is 100% lead (look at the pic attached, the bit at the bottom is the end of the fiberglass where the keel gets bolted to). You did not include a photo, so it's not clear where your bulge is? 

Assuming it is in that first 8 inches or so, the famous 'Catalina Smile' that your vessel likely had fixed many years ago (Catalina fixed the problem they'd created by using a piece of marine plywood under the keel step in about 1985 I believe) could be the spot where the ingression of water is happening.  Any lower and, per the other commenter's POV, it might just be water under a layer of fiberglass. 

Regardless, unless there's an unusually substantial leak THROUGH the original repair to the 'smile', it would be tough to imagine that the leak is large enough to risk the sinking of the boat.  Any one with the original stuffing boxes has regular drips into the bilge and all keel-stepped masts always have a regular influx of rainwater down the mast and into the bilge. 

On a dry day I would wet-vac out the bilge water and lay down with a flashlight and see if you can witness the water leaking in. If you don't see anything immediately, drying the bilge out with a towel and then laying down a series of sheets of paper towel from as far forward as possible back to the last bilge section might give you a clue as to the point of origin (I suspect the mast).  Not that to access the forward-most keel bolt you have to remove the floorboard in the V-berth (not an easy task -- best put back in with the screw holes bunged, sanded and varnished, then two-sided velcro from Home Depot to hold it back down). 

At the end of the day, I wouldn't spend any money on any serious repair like a haul-out, or air injection, etc., until you get a professional surveyor's opinion after you identify the source of the leak. 

Hm. Again, you'd have to suck the bilge dry and test to see if there's a ingress of water at the bolt.  If so, remove the nut and washer, use an underwater caulk around the bolt and replace the washer and nut and crank it back down to appropriate foot pounds of pressure.  Problem solved.  However, I've never heard of a blister in lead.  Maybe in the fiberglass that was faired over the lead keel at the top edge.  Still, always possible that water has made its way up through there and up the bolt.  Not an issue for the safety of the boat, however. 

Molding_Process_handout_v1.pdf (marskeel.com)
There may be a skin over the keel that blistered but the lead keel is not hollow it should be solid lead with keelbolts a friend of mine had to have his keel replaced after a grounding.