I have used actual blankets during some of the most bitter cold single digit nights. I would put the blankets on loose a few hours after dark and take them off before the sun touched them. There was perspiration that was frozen on the blankets from the hive. I had plenty of ventilation.

I know some people wrap hives in really cold climates. In santa fe the temperature fluctuates so much during the winter it might cause more problems than is solves. So I don't mess with insulating them but I do make sure they have a sizable wind break and are placed somewhere they get early morning sun, as much as possible.