Charles Ellicott Commentary Job 3:5

"Let darkness and the shadow of death claim it for their own; Let a cloud dwell upon it; Let all that maketh black the day terrify it." — Job 3:5 (ASV)

Stain. — Literally, redeem — i.e., claim as their rightful inheritance. This other meaning is also present in the word, as in Isaiah 63:3 and Malachi 1:7.

Blackness of the day — i.e., preternatural darkness, inopportune and unexpected darkness, like that of eclipses, etc.