Poe uses his typical unreliable narrator in "The Black Cat." Other Poe stories such as "The Cask of Amontillado" and "Tell-Tale Heart" use the same technique in which a mentally unstable person narrates his crime(s). "The Black Cat" begins with the narrator's confession that he is penning the tale before his death (execution). As he flashes back to the events which lead to his imprisonment, he constantly contradicts himself. For example, he swears that he loves animals and his wife, yet he ends up killing both in the course of the story. Additionally, he goads the police when they come to his house so that he can outsmart them, but the cat ends up causing his downfall.
In regards to the mood of the story, the unreliable narrator technique provides the reader with suspense and horror. When an insane person is telling others of his deeds, the listener has no idea what to expect next or how far the narrator will go with his misdeeds.