Rivers West sells a couple different sized fleece blankets that are waterproof, windproof, and puncture/tear resistant (barb wire tested). It comes with a damn fine warranty as well. Their Artillery blanket looks ideal for what I'd want, but if you need something smaller, their field blanket or their sniper bag might be better. Prices have gone up due to their success and the fact that they have no competition right now.
http://www.riverswest.com/hifi.htmlBass Pro shops has carried their product off and on. RW used to sell direct, but it looks like they've turned their on line sales over to Cascade direct, likely a retailing offshoot that took over the entire retailing on line aspect of the company.
I've got one of their blankets, and not only is it an effective outdoor blanket/ground cover, but it is also the wife's favorite "curl up on the couch and nap" blanket. Mine's 4 years old and still has no holes, no fraying, no problem. I also have a hunting outfit made from the same sort of fabric but in realtree mossy oak print that I absolutely love for the foul weather hunting.