The Burlap Sack in its spawn state is a 6 slot item, which can normally be cut into Burlap Strips or crafted into a Courier Bag.
After the Burlap Sack is used as a Blindfold, it becomes a 4 slot item. It can no longer be used in crafting. It also now possesses an insulation value. It has a different texture when appearing in the ground or in the inventory.