black as Newgate's knocker

A vivid British simile meaning completely, utterly black — as dark as the famous iron knocker on the gate of Newgate Prison, London's most notorious jail. The phrase dates back centuries and trades on Newgate's grim reputation: everything associated with it was dark, forbidding, and heavy. Used to describe anything from overcast skies to scorched toast to a blackened cooking pot, it has the dour wit characteristic of working-class London speech, and still pops up in older British writing and regional dialogue.