Jamieson, Fausset and Brown on Job 3:5

5. Let ... the shadow of death--("deepest darkness," Is 9:2).

stain it--This is a later sense of the verb [Gesenius]; better the old and more poetic idea, "Let darkness (the ancient night of chaotic gloom) resume its rights over light (Ge 1:2), and claim that day as its own."

a cloud--collectively, a gathered mass of dark clouds.

the blackness of the day terrify it--literally, "the obscurations"; whatever darkens the day [Gesenius]. The verb in Hebrew expresses sudden terrifying. May it be suddenly affrighted at its own darkness. Umbreit explains it as "magical incantations that darken the day," forming the climax to the previous clauses; Job 3:8 speaks of "cursers of the day" similarly. But the former view is simpler. Others refer it to the poisonous simoom wind.