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I think you may be referrng to a short story called "Pets," which was in a short story collection called "What Do Fish Have to Do With Anything?" by Avi. The story focuses on a girl who had two cats, and the white cat was a mean (spoiled and sort of evil) cat named Angel, and the black cat is a nicer one named Shadow.
The plot rings a bell, but the name doesn't cooperate with Google, so I can't check. That I can't find it here on TV Tropes doesn't help, either... I like how it reverses the typical stereotypes associated with black and white cats, though.
Google Books has the book available through preview, the short story should show up here: https://books.google.com/books?id=EJLfBd4MtsQC&lpg=PP1&dq=what%20do%20fish%20have%20to%20do%20with%20anything&pg=PA119#v=onepage&q=what%20do%20fish%20have%20to%20do%20with%20anything&f=false
(If it doesn't work, its on page 119).
I'm trying to remember a story from my English textbook in... Middle school/junior high, I think? I remember that there was a black cat and a white cat, I think the black cat was nice and the white cat was mean, and one of them (I think the white) was named Angel, I think. That's about it, though...