I use a wool blanket with my sleeping bag...

When hammock camping in the cooler seasons I put the wool blanket under my sleeping bag to compensate for the compression of the bag's loft.

When winter camping I use the blanket instead of or in addition to my sleeping pad.

Coldest I have been out in is -21F, in a tent.
From the ground up - Thermarest, Wool Blanket, winter mummy bag, summer mummy bag inside, with a fleece liner inside that. I was comfy and did not even melt the snow under my tent.

Don't put the blanket over your sleeping bag though... That was a very uncomfortable night.