Mace blades are the membrane that surrounds nutmeg. The dried nutmeg seed is found inside the nutmeg fruit. The web-like structure that surrounds the nutmeg seed breaks apart into slivers and those called blades.
Whole mace blades are often used to add flavor to soups and stews, but much like bay leaves they are removed from the dish before serving. Mace blades give a nice warmth and lemony sweetness to dishes. A little goes a long way.