I agree with JMS, don't blanket, the skin needs to breath. If your horses are in acold climate, this time of year, you can carry out a small bucket of warm water and a rag. After removing some of the top layer of hair, scrub with betadine scrub and rinse off with the warm water. Also do not use straight iodine as it can burn the skin, and don't clip under any scabs as it will cause pain. After the scabs start to loosen, should take a few days or up to a week then you can try to dislodge them carefully, but if you find any resistance just leave them until they loosen.
I once bought a colt with rain rot all over his back and hips, it took sometime, but was worth it.
Also once the weather is warmer you can bath and clip all the areas that were effected and treat again, to help with those awful black marks that appear after.