The fire itself in "The Black Cat" is not immensely significant, but its effect is. On the very night that the narrator hangs his once-beloved cat Pluto, his house catches on fire. The entire house with the exception of one wall is destroyed. When the narrator approaches a crowd gathered around the remaining wall, he notices that a figure on the wall, almost as if an artist had created it, is drawing their attention. He states that it is
"the figure of a gigantic cat. The impression was given with an accuracy truly marvellous. There was a rope about the animal's neck."
The strange phenomenon is simply that one wall remains which bears a striking, supernaturally created image of a hanging cat. The narrator sees this incident as his dead cat pointing him out from the grave, and it causes him to slide further into his insanity.
As a side note, the fire adds to Poe's hell motif in this story. His first cat bears the name of the god of the underworld, and the fire creates a sense of certain damnation for the narrator's actions.
Doesnt the fire cast a shadow of the Cat causeing it to seem bigger?