(Isa. 50:3; Ex. 10:21)
In describing the darkness as a covering of “sackcloth” … Isaiah presents a double picture. At the second coming of Christ, the heavens will be darkened, and the wicked will mourn in sackcloth. By closing with this symbol, Isaiah reminds Israel both of merciful redemption and the consequences of refusing his invitation.”
(Victor L. Ludlow, Isaiah: Prophet, Seer, and Poet [Salt Lake City: Deseret Book Co., 1982], 421.)