Hastings' Dictionary of the Bible (1898)
DARK'NESS. " Ex 10:21. This may have been occasioned by a thick, heavy vapor, or other sensible change in the atmosphere, which caused an entire interception of the sun's rays. It was evidently miraculous, and the dread and terror it inspired are vividly described. Ex 10:22-23. So of the darkness that shrouded the earth when our Saviour was put to death, Luke 23:44-45; it was manifestly miraculous, as no natural eclipse of the sun could take place at that period of the moon.
"Darkness" is used in a metaphorical sense for ignorance or sin, John 1:5; Rom 13:12; Eph 5:11; for misery, Isa 5:30; Isa 59:9-10; for the final doom, Matt 8:12, God is said to dwell in the thick darkness. Ex 20:21; 1 Kgs 8:12.