GRC makes some good points. You're always better off using actual acoustic paneling and tiles than blankets, but blankets are better than nothing. Another thing he didn't mention is that you can achieve some good things if they're a bit bunched up, rather than pulled flat. I would be more inclined to pick up some Auralex and do a mixture of Auralex and the blankets. I usually prefer heavy curtains or fabric hanging behind my monitors, near the corders of the room, with Auralex Foam on the side walls and ceiling above the listening position, with diffusion in the back of the room.
Emmett