Onyx Pikmin are onyx-colored, not-made-of-onyx pikmin able to avoid detection from night predators. It is because they are black and the night is black, and unlike what others would desire these do not have illumination abilities. Illumination would only serve to make Onyx Pikmin stand out and be easier to see. Obviously, this would eliminate their night resistance, and keeping pikmin alive is more useful than being able to see.