A blanket is a blanket, sounds like you are talking about a duvet cover. I've never even seen a duvet or duvet cover in real life, they're not common in America (at least not in my part). In fact, I didn't even know duvet/duvet covers existed until I started this job because I occasionally see them being imported and had to look them up in a dictionary.
Usually a bed here is a blanket or comforter, then a top sheet, then a fitted sheet on the mattress. In hotels/motels you'll sometimes see a comforter AND a blanket with the two sheets.
(Based on photos of duvets) I think a comforter is somewhat like a thinner duvet with a non-removable cover.
As someone else said in this thread, when I heard 'blanket' and 'zipper' together the only thing that came to mind was a sleeping bag.
