With my winter blankets I use a pressure washer. It does a great job of getting even the worst mud off. I actually just did it 2 days ago :). If you don't have one yourself you can just bring it to the car wash. I've done that before. I got a few looks, but it got the job done.
I have never tried washing it in the machine. I figure it is just easier to pressure wash. I am more worried about my machine getting ruined rather then the blanket. One of our boarders brings hers to a dry cleaner in town. The woman that owns it has horses, so she does peoples blankets. It's reasonable too only like $14, so you could try calling some dry cleaners.
Hope that helps!