I haven't used it on horse blankets (yet)...but I have for Snowboarding gear.
I've used a brand called "Camp Off" (I think it's for tents and such) and another brand that I bought at Big5 Sporting Goods (for generic clothing waterproofing). They both seemed to give the same outcome on my jacket and pants. It worked really well; I even tested it out by pouring a bowl full of water on the pants and it all just rolled right off!
Be sure the item is clean and totally dry before applying the spray. When I did mine, I did it in multiple layers to make sure I really got it covered. And let it dry for at least a day or two afterwards. You'll probably need a few cans for a whole horse blanket!