Darkness" is a state without light. "Light" is truth from the Lord, so "darkness" represents a state where truth is lacking. Here's a cogent passage from Arcana Coelestia: "The person who has yet to be regenerated is called 'a void and an empty earth', and also 'ground', in which no good or truth at all has been sown - 'void' where there is no good, and 'empty' where there is no truth. Consequently there is 'thick darkness', or stupidity and lack of knowledge about anything that has to do with faith in the Lord and so anything that has to do with spiritual and celestial life. (Arcana Coelestia 17)

In Genesis 29:17, this signifies that the affections of external truth have feeble and wavering understanding. (Arcana Coelestia 3820)

In Numbers 13:18, it signifies being without supporting truths. (Arcana Coelestia 6344)

In Psalm 6:2, weakness occurs in a prayer of the Lord to the Father when He was in the last state of temptation, which is despair. (The Inner Meaning of the Prophets and Psalms 256.)

In Psalm 109:24, it signifies a lack of nourishing goods and truths. (Arcana Coelestia 9050[7])