I suggest you go to the fabric store right now(On sale) and get two six foot lengths of fleece and then sew them together. Lighter than wool blankets, warmer than them and dries quickly if they get wet. Total cost was about $15. Packs about the same...which isn't great, but definatly lighter.
I take this with my cheap 20 degree sleeping bag and when combined with my self inflating Therm-a-rest, I'm quite comfy into the mid teens.

Oh and another thing, I lay the fleece blanket over the top of my sleeping bag, not inside or under.