Dark power (Kuroi chikara) is what the technique catalog calls the lock at the edge of death, or when the blade itself is dying. The color goes black. The output jumps. Chihiro’s Mei: Shred is black lightning on a dying Cloud Gouger. Samura’s Suzaku turns black when he spends his life; those flames stall Magatsumi’s wide drain long enough for Chihiro to leave. Magatsumi’s ordinary darkness is vitality as a diet, not a special mode.
Not a transformation gauge
True Realm is intent locking with steel. Dark power is that lock spent when the body or the sword is already ending. Reddit and YouTube recaps collapse the two. The chapters keep them next to each other: Honryō is the brief, black is the cost. We do not invent a third wartime blade’s black form. See True Realm, catalog, Tobimune.