total darkness; utter darkness
A literary yojijukugo meaning pitch-black darkness, as dark as a moonless night. Rare in modern conversation; mostly encountered in classical or poetic contexts.
Inside the cave it was pitch-dark, and I couldn't see a step ahead.
如法 is used adverbially here in the sense “thoroughly” or “utterly,” modifying 暗夜 “dark night.” The compound therefore denotes complete darkness.