Kuro (涅) is Enten’s first named technique. A black goldfish paints arm and edge. The flying slash is sized to the fish called. Kuro: Shred (涅千) is many small black fish, many small cuts, more spirit, less strain on the body. The fish can be moved as scouts and tools.
Not a transformation
True Realm is later and means Magatsumi’s death. Kuro is the daily language: the bowl made into a cut. Chihiro spends it empty-handed once spirit has left the body as a shape. Volume 11’s snow jacket still shows Aka and Kuro because the kit is the face of the seventh blade. Official chapters: VIZ / MANGA Plus.