The mood of "The Black Cat" is one of foreboding and menace. The speaker immediately insists that he does not expect us to believe him, that he is not a madman, and that he is scheduled to die tomorrow—to be executed for his crimes. He wishes to "unburden [his] soul" to us, and so it must be something terrible he is about to confess. These statements, alone, are enough to set the mood of the text, to make us feel the anticipation of wondering what awful acts the speaker is about to unfold before us.
However, before beginning the story, the speaker addresses the "horror" of these events, describing them as "phantasm" that differs wildly from the natural and commonplace. He says that he believes that some greater intellect than his own might read his account and develop an understanding of these events and how they can be explained away logically. These statements help us to understand that the narrator himself believes that there is something supernatural at work in his story, that there are things that have happened to him which he cannot explain logically. Such a claim only increases our feeling of wonder and our sense that we are about to hear a strange and disturbing tale.