Howdy Sir William,
Sometimes you get blisters from an extensive shooting session, and the culprit is the firearm itself. Hold the revolver in question, and look at where your blisters contact the gun. Are there any edges on the gun that you could take a fine tooth file or emory cloth and smooth out? Tyler's T-Grip (?) is an aluminum filler piece that can be purchased to fill in the area above and behind the trigger guard while still using the older period grips, that might also help. When gunsmiths prepare a carry package, they often blend, or ease all the edges on a gun that might contribute to blisters.