The Demonic Sword began to shake while Noah held it. That phenomenon didn't come from the accumulation of dark matter in its structure. It was a form that Noah had added to give an active task to the higher energy generated by the blade.
Martial arts required forms to consume "Breath" and use it to produce stronger effects, and the same applied to the higher energy. Noah had already seen from Snore's dark beam how the dark matter reached its highest destructive potential in its violent shape, so he created a move that imitated the effects of its innate ability.
Noah had it easy in that task. He had invented Snore's dark beam in the end. Translating the movements of its organs into a form wasn't hard. The problems had begun when the dark matter started to diminish the effects of his darkness.
Still, Noah had used the years while waiting for Night to wake up to test and polish that new move. That martial art was already perfect in the trial.