Pluto's eye in another fit. The second black cat he adopts is never enough like Pluto for him; this also drives the narrator into a rage.

The antagonist of a story is the character who is in conflict with the protagonist, and the protagonist is the main character of the story. The main character in "The Black Cat" is the narrator, and although the narrator is clearly in conflict with himself throughout the story, he is also, from his perspective, in conflict with his pet cat.

At the very beginning of the story, we are told that the narrator and his black cat initially get on very well together. The cat becomes the narrator's "favorite pet and playmate." However, one night, coming home drunk, the narrator decides to cut out one of the cat's eyes. From that moment on, understandably, the cat becomes wary of its owner. Not long after, the narrator decides to kill his cat.

One interpretation of the events following this grisly act is that, after the cat's death, the spirit of the cat begins to relentlessly haunt the speaker. It is thus the cat's death which marks the moment from which the narrator and the cat are in real conflict. To the narrator, the spirit of the dead cat becomes the antagonist and is no longer merely the victim.

At first, the dead cat appears to the narrator only as a disconcerting "apparition" on a wall, but then, in order to better torment him, it possesses the body of a second, almost identical cat. In the form of this second cat, the spirit of the first cat haunts and torments its former owner, inspiring in him unbearable feelings of "terror and horror." The narrator comes to know the second cat as a "brute beast" and his "tormentor."

Eventually, the cat gets its revenge on the narrator and thereby fulfills its role as the antagonist of the story. It is the cries of the cat which alert the police officers to the whereabouts of the narrator's murdered wife and which thus condemn the narrator to his death.

There is no dialogue in Edgar Allan Poe’s short story “The Black Cat.” Dialogue consists of verbal interaction between two or more people. The narrator of Poe’s story sits alone in a prison cell, his execution imminent, and pens the history that has brought him to this foreboding place. He relates the tale of his relationships to his wife (only in passing), to two large black cats, and to the instrument of his doom: alcohol, or as he calls it early in his narrative, “the instrumentality of the Fiend Intemperance.”

There is no verbal interaction quoted throughout the narrative, and the only spoken words of note occur near the story’s conclusion. The narrator, confident that his evil deed—the brutal murder of his wife and the body’s internment behind a wall—will remain a secret, boasts to the investigating police who have returned to his abode to again search the premises for signs of criminal activities that the structure in which he resides is especially well-constructed:

“Gentlemen,” I said at last, as the party ascended the steps, “I delight to have allayed your suspicions. I wish you all health and a little more courtesy. By the bye, gentlemen, this—this is a very well-constructed house,” (in the rabid desire to say something easily, I scarcely knew what I uttered at all),—“I may say an excellently well-constructed house. These walls—are you going, gentlemen?—these walls are solidly put together”; and here, through the mere frenzy of bravado, I rapped heavily with a cane which I held in my hand, upon that very portion of the brickwork behind which stood the corpse of the wife of my bosom.

That’s it; there is no verbal response noted by the narrator from the police officers who hear the horrific screeches from the cat mistakenly entombed behind the wall that heretofore concealed the decaying corpse of the narrator’s wife. The investigating magistrates simply respond to the mysterious and horrifying screeches by tearing down the wall and discovering the corpse and the cat.

“The Black Cat” is told from the first-person perspective of an unrecovered drunk whose grasp on sanity had long since slipped away. It is told entirely in the first person with no dialogue provided, save the verbal communication at the story’s end that results in the narrator’s arrest for the murder of his wife.

Interesting question! Poe enhances the perverse nature of this story by presenting the narrator as both. By his sordid actions, the narrator is evidently the antagonist; but he solicits the reader's attention, then sympathy, as if he were the protagonist. He even goes as far as to justify his acts as if he himself were a victim of circumstance.

This reversal of roles makes the reader an accomplice of sorts in his crimes, as if he naturally approved of the narrator's acts. 

Although most people think of a protagonist as the "good guy" or the hero of the story, a protagonist is simply the main character. I have provided a couple of links explaining what a protagonist is; the first is a literary site that explains the concept in terms of literature only, and the second is just a dictionary definition of the word. 

In The Black Cat, the protagonist is the narrator, and he is certainly not a good guy or a hero. In fact, he is quite the opposite, as he ends up being abusive both to his wife and his pets when he is drunk (which is often), and he ends up killing both his favorite cat and his wife in rather gruesome ways. This narrator is a perfect example of how protagonists are not necessarily always good.