The black light, also known as ultraviolet light, can be beneficial if it is used as supplemental lighting.
UV light harms DNA in all living things by destroying proteins and lipids and causing cell mutation or apoptosis. Nevertheless, these cell mutations and death can be advantageous while cultivating cannabis.
Cannabis plants produce resin in response to UV light because resin glands are a defence mechanism for the plants when it senses the need to defend themselves.
You won’t be able to enhance resin production if the plant is too young and incapable of manufacturing resin. Thus, the black light is usually used as additional lighting during flowering till the final stages of the grow cycle, when it will have the greatest effect.