Solder is used to join the barrels/ribs together. I wouldn't go so far as to say hot bluing WILL ruin the juncture, but it CAN. I'm guessing it depends on the melting point of the solder used and the temperature of the bluing solution.
A quick way to tell if the barrel/rib juncture is good is to remove the barrels from the frame, hang them down by the underlug from your finger (loosely, not touching them anywhere else), and give the barrels a good rap with your finger down towards the muzzle end. If this makes a nice sharp "ring" they are still solidly joined. If it makes a dull "thud" it means the barrel/rib juncture is loose.
