Rambo and Schneiders both make blanket wash detergents that preserve waterproofing; and you can wash it in a ultra-capacity or commercial machine at a laundromat. Just wipe out the machine when you are finished, so they don’t ban you from the premises in the future. I use cold water and low to medium spin cycle, then hang to dry. When it is dry, I often spray around the neck opening and along the back area with “Camp Dry” (available at Wal-Mart and outdoor stores) to bolster waterproofing.
There is a limit to how much you can expect waterproofing to last, especially if the blanket is a few years old and has been exposed to a lot of wet weather. Eventually, the fiberfill is also compromised. When your horse is not warm to the touch under the blanket, you know it’s time to get a new one.