Pro tip. Regular Iron does not work.

The misconseption comes from the fact that the fae have the power of decay. So they rust metals. However cast iron was historically very high carbon steel. So it dosent rust. But its very brittle so we generally dont use it any more. And genarally havent used it since bloomeries appeared 3000 years ago. Black iron is iron that has grown a protective layer of black oxide may also keep them at bay for a while.