There are some fairly obvious similarities in these stories by Poe, and there are even a couple of similarities between "The Black Cat" and "The Cask of Amontillado". In terms of the darkness of these works, it is probably easiest to assume that they were an outgrowth of extreme depression and an obsession with death that probably related to several painful losses experienced by Poe: his mother when he was a child, his foster mother when he was a young adult, and his young wife. In any case, the "The Telltale Heart" and "The Black Cat" are both told by narrators who are supremely confident in their own sanity, and say so. In "The Telltale Heart", the narrator kills an old man because he doesn't like the old man's eye; in "The Black Cat", the narrator cuts out the eye of a cat who has angered him. When the narrator kills his wife in "The Black Cat", he "buries" her in a crypt type thing underground in a manner very similar to the burying alive of Fortunato in "The Cask of Amontillado". The narrator in both "The Telltale Heart" and "The Black Cat" calmly lead the police on a tour of the area where their murder victims have been left, while the narrator in "The Telltale Heart" gives himself away when he thinks he hears the dead man's heart pounding; the narrator in "The Black Cat" gets caught when a loud cry emanates from behind the wall where his wife's body is.
In both stories, as well as "The Cask of Amontillado", the narrator seems calm, cool, and collected, perfectly normal as he discusses his problems matter of factly, In "The Telltale Heart", the narrator mentions that he actually kind of likes the old man he's going to kill; he has no quarrel with the old man, and he doesn't want his money--he just doesn't like to see the eye. As he begins to detail his plan, watching the old man sleep, night after night, one begins to realize how very sick the narrator is--capable of evil brutality against someone who is essentially helpless. In "The Black Cat", the narrator discusses his problem with alcohol in much the same way, with no real acknowledgement that it is a problem; his matter of fact approach is mildly misleadng until it becomes apparent how much hatred he is capable of expressing against both people (his wife) and animals, like the cat.