'Pitch dark', or 'pitch darkness', is an expression used to indicate an absolute absence of light. Of course, this condition, is rarely achieved in nature and the phrase is used in practise to mean 'extremely or intensely dark'.
Pitch is the sticky, resinous, black or dark brown substance that is obtained as a residue from the distillation of wood tar or turpentine.
'Pitch dark' derives from the earlier expression 'pitch black', which in turn derives as a variant of 'as black as pitch'. The latter variant was first recorded in several 14th century texts. 'Pitch black' is known from John Marston's satire The Scourge of Villanie, 1598:
Hath drawn false lights from pitch-black loueries. [a louvre was a domed roof turret]