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)

3949. 'And Jacob came from the field in the evening' means the good of truth in a state of good, yet also in obscurity such as envelops the natural. This is clear from the representation of 'Jacob' as the good of natural truth, dealt with in 3669, 3677, 3775, 3829; from the meaning of 'the field' as the Church as regards good, dealt with in 2971, and so as good itself; and from the meaning of 'evening' as obscurity, dealt with in 3056, 3833.