These blankets are relatively easy to clean.
Just hose them off with a hose, or a waterblaster, to get excess mud and hair off, and hang out to dry.
If they are very dirty on the fabric side, then you can give them a scrub with washing powder, and hose/waterblast them off and hang out to dry.
They do take quite a while to dry however, as the wool felt soaks up the water!
I have two, so that I always have one spare when the other one is drying.