Go to your drug store and pick up 2-3 hot water bottles and fill them with hot water and put them under the blankets for Blackie or you could always plug in a heating pad, keep the setting on low or medium (depending on how thick the blankets are) and this should provide some extra warmth. Also hay is a good insulator, it is really cheap but provides a lot of insulation for a kitty.

My Rainbow Bridge baby Cleo was a feral that I helped out, and this is what I did for her on many a cold day down here in Connecticut. Good luck.