You might want to consider this question by thinking of how the black cat itself is described throughout this excellent tale. Remember, of course, that the story is told from the first person point of view, and so it is the murderous and slightly insane narrator himself who reports what he sees and what happens, and it is clear that his relationship with the black cat is what leads to his act of murder and his ever-more tenuous grip on reality and sanity.
Thus it is that tension is sustained throughout the tale by descriptions of the black cat as follows:
I started, hourly, from dreams of unutterable fear, to find the hot breath of the thing upon my face, and its vast weight--an incarnate Night-Mare that I had no power to shake off--incumbent eternally upon my heart!
References made to the black cat which describe it as a "thing" of demonic proportions, an "incarnate Night-Mare," serve to heighten our interest and the tension of the tale as we wait to see what will happen and how the curious relationship between the narrator and the cat will resolve itself.