The character Black Dragon, formally named Dragblacker, serves as a clone of the dragon Dragreder. It functions as the Advent Beast contracted to Kamen Rider Onyx, provided by the entity Eubulon.

Dragblacker possesses the distinctive ability to interact with stressed Riders through their dreams, manifesting spectral visions of its affiliated Rider within nightmares. This capability surfaces when it appears in Kit Taylor's dreams, projecting an image of Onyx. While other Advent Beasts reportedly induce nightmares in their own Riders, Dragblacker exhibits this ability toward a Rider not initially under its contract.

In combat, Dragblacker engages through specialized techniques activated by cards slotted into the Blackdragvisor arm device: Attack Vent summons Dragblacker for direct assaults; Sword Vent conjures a broadsword featuring black dragon motifs; Strike Vent attaches a draconic gauntlet to launch bursts of black flame; Guard Vent provides protective arm guards; Final Vent enables a coordinated finishing move where Onyx executes a flying side kick while Dragblacker paralyzes the target's legs with dark energy and amplifies the kick with dark flame propulsion; Link Vent unleashes the combined power of all thirteen Final Vents.

Initially, Dragblacker breathes dark flames during nightmare manifestations. After Kit Taylor assumes the mantle of Onyx, its breath weapon shifts to conventional orange flame. Unlike its counterpart in Kamen Rider Ryuki, Dragblacker survives all depicted conflicts and is explicitly characterized as a benevolent entity among Advent Beasts.