In Exodus 3:12, this signifies a confirmation of the truth that the Divine proceeded from Himself. (Arcana Coelestia 6870)

(References: Arcana Coelestia 1039, Genesis 9:12, 9:13, 9:17)

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)