Well, I think it is mostly the whole "don't let a black cat cross your path or it'll bring you bad luck" thing. And their close association with witches and Halloween. I don't know what all countries are superstitious about black cats, but there is a certain amount superstition here. It's silly really, because there is not a sweeter kitty than my Pest.

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Personal servant for Pesto
http://www.angelfire.com/80s/pestoswitzer/main.html
http://www.angelfire.com/pq/coolcatclub/main.html