To clarify, this is not to cure the black parts of the comb. Those are gone and will fall off as previously stated. This also depends on your coop setup and how much you want your chickens to be acclimated to the cold. There is a sliding scale of frostbite management. On one side is the...
That's definitely frost bite. A coop-safe heating lamp might help. You can also massage their combs with Vaseline a couple of times per day to get blood circulating.