What are you growing? Autos? Photoperiods? If they're autos, I would increase the number of hours they're getting light as the lights should generate enough heat for them... When I grow in the winter, my autos are under lights 24hours a day because I, too, cannot keep them warm enough otherwise. An electric blanket isn't going to give off enough heat - you really have to be touching those to feel warm and I doubt you'd want to actually drape it over your plants.. The plants would benefit from more than 12 hours of light anyway - more light = bigger buds...
Good luck!