In the 2nd season, "Dark Power" was the Black Moon Clan's main strength and source of their own abilities. It appeared as black lightning in its normal state.
Each of the Spectre Sisters tried to infect Crystal Points in 20th century Tokyo with Dark Power, to destroy the foundation points of the future Crystal Tokyo. Destroying the points weakened the defensive powers of the Sailor Senshi in the 30th century guarding the city, thereby allowing the Black Moon Clan to take over Crystal Tokyo. However, they failed in their tasks.
Later, Esmeraude used Dark Henge which she would place at specific Negative Points in Tokyo. When Dark Power was supplied, the Henge would grow, creating a Dark Gate at the Negative Point.
Almost all of the Black Moon Clan members controlled Dark Power in its natural lightning state. But when Koan summoned Dark Power, it took the form of blue fire.
Petz attacking with Dark Power in her Dark Thunder attack.
Koan summons Dark Power in the form of blue fire.
Prince Demande summons Dark Power using his earring.
Black Lady summons Dark Power.
The Black Crystal in Tokyo is being supplied with Dark Power, enlarging it.