Green’s Dictionary of Slang

black as... adj.

[mid-19C+] used in var. combs. to mean extremely black, usu. of darkness, occas. of race.

In phrases

black as Newgate (adj.) (also black as Newgate wall, dark as Newgate) [Newgate, 19C London’s main convict prison and the site of many executions; its reputation was fig. black] [early 19C]

1. of an expression, frowning, glowering.

2. dirty, e.g. of a garment.

black as Newgate knocker (adj.) (also black as Newgate’s knocker) [see prev.]

1. [late 19C] (also dark as Newgate knocker) of a night, very dark.

2. [1980s] very dirty.