as black as newgate's knocker

The phrase "as black as Newgate's knocker" is an idiomatic expression used to describe something or someone that is extremely dark or black in color. Newgate's knocker refers to the large iron knocker or door handle found on the gate of Newgate Prison in London, which was renowned for its dark and grim atmosphere due to its notorious history as a prison. Therefore, when something is compared to Newgate's knocker, it implies that it is as dark or black as the door handle of that infamous prison.