Hose the blanket down thoroughly to remove mud, hair, urine & feces. Hang over a fence to dry thoroughly. Then find a laundromat that is attended and has at least a 50# front loader. Be prepared to run the machine a second time when you are done to remove all dirt, hair and smells from the machine.
Many laundromat owners do not allow horse blankets to be washed at their estabilishments. Horse blankets are very heavy when wet, usually very smelly from urine and poop, and tend to clog the drain in the washer with mud and hair. This is why you should pick an attended laundry; if the machine has a problem, there is someone there to help you out immediately.