(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."

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.”