What is the significance of the fire in Edgar Allan Poe's story "The Black Cat"? There's a symbolic phenomenon that I don't seem to understand.

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.