Psalms 91:6

6. Pestilence that walketh in darkness The usual idea of דבר , pestilence, is here preserved, a mode of destruction, whether by sickness or other divine judgment, which is sudden, secret, and without warning. (See on Psalms 91:3.) It “walketh in darkness,” like the angel of death. Amos 4:10.

Destruction that wasteth at noon-day A description of a battle, and parallel to the “arrow that flieth by day,” Psalms 91:5. It is better to thus take Psalms 91:4-5, as one verse of four lines, as some do, and as the antitheses would justify. But קשׂב , destruction, seems best understood of the sirocco, the terror of all travellers and dwellers in and adjacent to the Arabian desert. In the two other places where the word occurs it is associated with “burning heat” (Deuteronomy 32:24) and violent “storm,” (Isaiah 28:2,) the known accompaniments of the sirocco. See on Psalms 103:16. Bishop Mant thus understands it, and paraphrases Psalms 91:6:

“Plagues that in the darkness waste,

Nor the noontide’s purple blast.”

See more on Psalms 103:16