"The Black Cat" concludes with the police discovering the narrator's crime: the murder of his wife. While irony is one of the most prolific literary devices used in the story, including the denouement, there are a few examples of symbolism as well.
Earlier in the story we can find obvious symbolism in the form of the second cat: its white marking takes the shape of the gallows, a reminder of the narrator's hanging of Pluto; its habit of sitting on the narrator's heart might symbolize the guilt and horror of his heinous crime against Pluto.
As for the symbolism at the denoument of the story, one critic sees the second black cat as a symbol of the narrator's hidden, evil self--the one he tries unsuccessfully to disguise from the reader and the police as well. While the narrator refers to the cat as a "monster" and "hideous beast" the term more accurately describes the narrator's own wretched, debased soul (R. Moore).
R. Moore. "The Black Cat: Critical Discussion." eNotes: The Black Cat. Ed. Penny Satoris. Seattle: Enotes.com Inc, October 2002. eNotes.com. 22 April 2010. <http://www.enotes.com/black-cat/critical-discussion>.