The son of Obsydian, the rock giant despot who ruled the Kingdom of Twilight in the other-dimensional realm of Darkearth, and the human geologist Katrina Marcum, Onyx learned he could switch between his human and rock form as a young child.

As an adult, Onyx stole the Ring of Midnight from his tyrannical father and fled to his mother's world of Earth. He arrived in Central Park, where he was mistaken as a monster by the population and was engaged in combat by Captain Defender and Pixie. After a brief battle, they decided to band together as the Keepers, and Onyx adopted the human guises of Jim Marcum and Blackrock.

Blackrock has increased height, mass and strength and is able to lift 30 tons. He can see into the ultraviolet portion of the electromagnetic spectrum. His rock skin is three times as resistant as concrete.

His goggles protect him from flash attacks by reflecting them back at the source.

He wears the magical Ring of Midnight, which allows him to fly eight times a day, project fields of darkness that blocks normal sight three times a day, and fire four beams of armor-piercing darkness. The ring does not work in darkness fields or at night, and recharges its lost charges every morning at dawn.

In darkness fields, he reverts to his human form of Jim Marcum, whose sole power is his ultraviolet vision.