I use blankets in my wall hangings all the time. One thing I've learned though, is to use blankets that are soft and flexible. I've seen some cheap, cheap blankets that feel like a stiff sheet of woven plastic. I once used a blanket that was loosely woven and once quilted and complete, it don't look smooth so I won't do that again.
But think of how durable a blanket batting would be!!
Peggy