Buy a big roll of white zinc oxide tape. Your local chemist should have it. Buy it at least 4-5 inches wide. Then use it to cover the skin of the heel, coming up the sides higher than the top of your shoe.
This stuff sticks like sh*t to a blanket so don't try pulling it off afterwards. Just let nature take its course. It will usually come off on its own after a day or 2. I suffer exactly the same problem and this works for me, even with a fresh blister. If I know I am in for some serious fast descending, I always use it else I'm in agony for days!