In "The Black Cat," the narrator begins the story by speaking directly to the reader. The narrator continues this practice periodically throughout the story. How does the author's use of this structure create mystery in the story?

Knowing that the narrator is speaking directly to you as the reader and giving only his perception of what happened leads to questions of whether or not he is a reliable narrator and lends an air of mystery to the story. In the opening of the story he even says he does not expect to be believed and that his “senses reject their own evidence,” so the reader cannot be sure of what is true. This uncertainty about the truth also adds a sense of mystery to the tale.
From the beginning of the story, we only have the narrator’s word that he was a kind animal lover early in his life. The only actions we see, starting with his confession of intemperate language and violence toward his wife, contradict that and add to the questions about what has really happened. The reader has no way of deciphering what actually happened.
Later in the story, we still have only dubious ideas of what led up to the murder of the narrator's wife. He claims to see an image of a hanged cat on the wall of his burned-down house, but he makes no mention of anyone else noting this image. Later, he says that the second cat had a white mark that was at first no particular shape but slowly took on the shape of a gallows. We have no way of knowing if the mark actually changed or if his perception of it changed.
Overall, this is a story of more questions than answers. As the story goes on, we learn that the narrator is sinking further into alcoholism, and his comments become more suspect until, by the end of the story, the reader is left unsure of whether this is a story of a man’s descent into alcohol-induced madness or a story of supernatural revenge.