My horses have always gone out in the day time and either locked in a stall a night or had a run-in. I have never been one to blanket my horses and I've taken two of my Keepers to ages 27 & 29.
The times have changed with my second group of Keepers. My two mid-20's fellas are now hard keepers and one of them has Equine Metabolic Syndrome.
I'm from the OH/PA border originally and know what "bone-chilling cold" really means - lol
If your hard keeper has to be out in rain and bone-chilling cold, I would blanket him with a water-proof blanket.
If you bring everyone in at night, that gives all of them ample time to dry and warm up again. If anyone is shivering and you're expecting more of the same weather the next day, I'd put a blanket on that horse too.
If you have time before you leave for work, rake a plastic brush or something stiff-bristled thru everyone's hair to fluff it up, after being matted down from the rain. A fluffy and clean horse is a warm horse
Hope this helps