If we are looking for "rational" explanations for the occurrences in "The Black Cat" or any story by Poe, we are missing the point somewhat. That a single wall would remain standing in a house consumed by fire is not unusual, but here, it functions as a mechanism of either supernatural vengeance against the man or (and this is probably the more basic fact in the story) a projection of the man's own guilt. Dark Romantic fiction inhabits a world in which the reader cannot be sure if the action is meant to be taken as "real," or rather, is a dream, a hallucination, or a self-created world in which the protagonist is playing out his inner demons. The gigantic image of a cat that appears on the wall is a symbol of the retribution to be inflicted upon the man. What's especially interesting is the convoluted explanation he gives for its appearance, stating (in all seriousness) that when the fire broke out someone must have cut the dead cat from the tree and thrown it through the window into the house in order to awaken him. Then, the man reasons, the action of the fire caused the shape of the cat to be impressed upon the wall, which just happens to have been freshly plastered and is therefore wet and susceptible to the creation of this huge cat image.
All of this serves the purpose in the story of enhancing the general atmosphere of terror but also showing the emotionally bizarre and distorted mental world of the narrator. The man knows that he is being punished for his crimes, but he persists in rationalizing these lurid occurrences with natural "explanations," showing the degree to which he's in denial. Similarly, when the second cat, virtually an exact replica of Pluto, appears, he seems at first to accept this as a normal event and not a bizarre coincidence that can be explained only with recourse to the supernatural or, more realistically, to his own psychotic mental state. It is only when the white spot on the new cat changes its shape into that of a gallows that he finally yields to this fact of retributive justice. But even at this point, his behavior does not change, and the murder of his wife leads to the final catastrophe for the man.