The dark mirror is the darkness. Think of it as Yin and Yang, Cause and effect, balance, duality, etc.... One needs the other for it to exist, to have purpose. The darkness would not be here, if not for the light, and vice versa. Both give each other purpose and existence. I do believe the traveler and the speaker have an ulterior motive, hidden agenda, or there is something that they are hiding from us. It is quite obvious.