Board Thread:Questions and Answers/@comment-10797017-20130622101437/@comment-11051098-20131023220027

It's by far possible. But in that case, to what extent could darkness be an ability? Could it be something similar to Tôsen's Bankai, Suzumushi Tsuishiki: Enma Kôrogi, using darkness as a tool to fight, or as an actual weapon in itself?

Mugetsu appeared, besides turning the surrounding area pitch black, to also be the actual weapon that appeared to cut Aizen's body in half. Maybe an extension of the blade itself?