If the barn is not heated, then of course leave the blanket on! I would say a horse doesn't need a blanket when it is 40-50 degrees.. but it varies. Like my horse has a pretty thick winter coat, if yours is clipped or has a thin winter coat then blanket them at 40-50. Anything below 40 I always blanket, and when my horse was living in the unheated barn he had two blankets on 24/7 throughout the winter. (late fall and early spring, when it was a little warmer, he only had his newer one on.) He now lives in the heated barn, so his blanket comes off at night, the heat is set to 50 and keeps the body warmth of the other horses well, so he stays nice and toasty without his blanket. =]