My (usual) criteria for blanketing is this: colder than usual, wet/windy/colder than usual, or if my horse is wet after dark and it's below 50℉. The prediction for this past few days was for wet and unusually cold weather. And since it's been so unseasonably warm, I worried that my guys might be cold since they hadn't had time to acclimate. I shouldn't have worried.
At this moment, it's 39℉ with 0 mph wind. For my boys, both young, healthy, and hairy, this weather doesn't really warrant blanketing.