Horse Blanketing Question?

That's a big temperature drop in a short period. If your box stalls are big enough I'd tie two horses in each stall so they don't fight along with some huge hay bags within reach for each of the horses to work on overnight. If the horses' blankets are wet put them in the dryer and be sure that the animals hair is clean underneath when you put the blankets back on.

Take the blankets off so they can dry before it gets too cold. Give them a little food to raise their body temperature. If you notice they are shivering and are still wet then blow dry them a bit until they are more dry. Once they are dry they should be okay in the colder weather.