black as pitch
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(as) black as pitch
Very dark or black in color A: "I can't believe that Grandma's hair used to be black as pitch." B: "Well, sure—you've only ever known her with gray hair."
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
black as pitch
Totally dark, without any illumination. Distilling coal, peat, and other organic substances yield a very dark resinous residue called pitch (“pitch” and “tar” have became virtually synonymous). Anything that dark made a very good simile for “black.” The contemporary expression is “pitch black.” Those who chose to give the absence of light a theological cast would have said “black as sin,” while an imaginative Irish simile was “as black as the Earl of Hell's waistcoat.”
Endangered Phrases by Steven D. Price Copyright © 2011 by Steven D. Price