A poetic noun denoting deep darkness or gloom. Isaiah 50:3 uses it in a striking image of cosmic mourning: 'I clothe the heavens with blackness and make sackcloth their covering.' The word appears only here, though it derives from a verb meaning 'to be dark' or 'to mourn' (since mourning involved wearing dark garments). The parallel with sackcloth reinforces the funereal connotations—darkness as a sign of divine judgment or lamentation.