"The Black Cat" is a brutal story, where the home becomes a site of torture, terror, and murder. The man admits to abusing his wife and animals, but only goes into detail a few times. The first time involves a pen-knife.
A pen-knife is supposed to be used for sharpening the narrator's quill pen. Still, it is a knife and always has the potential to be used as a weapon. When the narrator uses the knife to "deliberately cut one of [Pluto's] eyes from the socket" the knife's potential is fulfilled (7). When we read the above lines we might get some kind of intense image in our mind. It probably provokes a variety of feelings.
When we understand the knife is meant to sharpen pens, the imagery becomes confused. The mind wants to see a pen where it sees an eye. Symbolically, the man is sharpening the cat's eye with his knife. Pluto learns to see that his beloved master is cruel and violent to the extreme. He also will experience a literal change of vision – from this moment on, he'll see the world through only one eye. Not coincidentally, the reader's eyes are sharpened at this moment as well. Crimes of violence we have hopefully never heard of before are revealed.
From King Lear to The Chosen, to Invisible Man, damage to a character's eye signals us to a changing vision in the story. By shaking us up with violence, damaged-eye symbolism might also put us in a space to experience changed vision ourselves. Most notably it might make issues of animal cruelty and spousal abuse more visible to us.
But it gets even deeper. As the narrator reminds us in the line following the one quoted above, he is penning or writing his confession. By making the man the writer of his own story, Poe creates a twisted double of himself, the real writer of the story. Working with the pen-knife, the story becomes an allegory about writing.
Writers often draw from the real life to write their stories. If they go too far, they can hurt the people they write about. There is also the possibility of hurting the readers, either with bad writing, or with good writing that gives readers bad thoughts or ideas. Writers are often conscious of the ability of writing to do violence to the vision of others.