I've slept under nothing but a 300 weight fleece blanket in summer and fall in the Pacific NW. I wouldn't recommend it, as most camping blankets are short a tad (I'm 6'0"), they aren't windproof (I sleep under a tarp), and they aren't that light.
Fleece blankets are warm, light, and comfortable, and I really enjoy the freedom of a blanket over a bag. But, there are better choices. I've since obtained an Integral Designs Prima Blanket, which is (off the top of my head) rated at about 50F, is 1-inch thick, is 28 oz, is filled with water-resistant Primaloft, is shelled (on the outside) with water-resistant Pertex, and, thanks to the zip-out hole in the middle, doubles as an insulated poncho. It's also black which facilitates drying in the morning sun. With a 5-oz rectangular silk liner bag (so I don't have to sleep directly on my sleeping pad) I'm comfy down to 45F.
This year I obtained a Jacks R Better Biker Quilt when I saw some offered at a great price. It's a 24-oz down blanket that's 1.5-inches thick. With the silk liner I've a comfortable 29-oz system good to 40F.
Both the Prima Blanket and Biker Quilt convert to bags if one opts for such a configuration. This option is nice for making a foot box when the temperatures are low enough to want to keep my radiating feet covered. These are both a lot pricier than a camping blanket, but I've found them to be worthwhile purchases.
My son has a poly-filled Fanatic Fringe quilt rated to 30F--very light and lots of loft, but it has a fixed and rather small footbox, which I find too confining.