(as) black as Newgate's knocker

dated Completely black; totally without light or color. Refers to the black iron knocker on the door of Newgate Prison in London. Primarily heard in UK. The basement gives me the creeps, it's as black as Newgate's knocker down there! Is this meatloaf supposed to be black as Newgate's knocker? How long did you cook it for? My hands were black as Newgate's knocker after working on that filthy engine all day.