As black as a Newgate's knocker

Meaning

Pitch black.

Origin

Refers to the heavy iron knocker on the gate of Newgate Prison, London. Several other similes are used to indicate blackness:

As black as a crow
As black as a raven's wing
As black as ink
As black as Hell
As black as Hades
As black as death
As black as the grave
As black as your hat
As black as thunder
As black as midnight
As black as pitch
As black as soot
As black as tar

... pretty much anything in fact, so long as it conjures up thoughts of darkness.

'Newgate's knocker' was a synonym for age and toughness (although not directly of blackness as far as I can tell) by the 18th century; for instance, in this piece from The Batchelor: or, Speculations of Jeoffry Wagstaffe, Esq, 1769:

Dear Jack, I wish your old dad would tip off [the perch, i.e. die], that you might come once more: damn it, he is as old as the knocker of Newgate, but I think as tough as a gad. [a bar of iron]

See other 'as x as y similes'.