To my mind, the most powerful symbol that there is in this chilling short story by Edgar Allen Poe is the black cat itself, which is obviously important enough to warrant being used for the title. Note how the story progresses it seems to symbolise for the narrator a constant curse that is always present and always dogs his every step - a reminder of his murderous instincts that he cannot forget or ignore:
And now was I indeed wretched beyond the wretchedness of mere Humanity. And a brute beast - whose fellow I had contemptuously destroyed - a brute beast to work out for me - for me a man, fashoined in the image of the High God - so much of insufferable wo! Alas! neither by day nor by night knew I the blessing of Rest any more! During the former the creature left me no moment alone; and, in the latter, I started, hourly, from dreams of unutterable fear, to find the hot breath of the thing upon my face, and its vast weight - an incarnate Night-Mare that I had no power to shake off - incumbent eternally upon my heart!
Note the way that in the narrator's mind the black cat comes to dominate him psychologically in a somewhat disturbing fashion. As we read on, it is his desire to kill the black cat that leads to his slaughter of his wife, and it is the black cat that at last condemns him to hell for his crimes, as the black cat reveals his murder and therefore condemns him to the gallows.