black cats are historically considered evil for several reasons. first of all, of course, black is supposidly the "devil's" color representing night and dark and shadow and all the rest of that bs. cats were sometimes seen as evil (regardless of color) because they were used as familiars by witches. and of *course* just because someone has an ability you don't understand or can't replicate yourself, it has to be an unholy power, right? so creatures that are both black, and cats, have two strikes against them already. enter the third (and only logically valid) reason they were 'devil-spawn':
black cats are rare. Unlike in many species, black is a *recessive* gene in cats. for a cat to show black it has to have *both* color-related genes black. and before cats were selectively bred to show various colors, patterns, and furs, random breeding of barncats and pets kept as mousers was statistically least likely to produce black cats of any other natural color. (actually that's not quite right--a pure white cat is as genetically unlikely. but in a quasi-feral state, these were not likely to reach maturity because their very visible coloring made them easy prey for predators.) but any "rare" or unusual coloration leaves room for interpretation: "why do we have a nest full of tabby and tortie kittens here, eight of them, mom's tortie, and *there's a black kitten here?* i've never seen one of *those* before... must be devil's work, a changeling creature". (in a time that believed in fairy and elvish changelings to explain behavior of children that weren't "normal" in thinking or ideas or looks, its a *small* leap to carry that over to creatures.
one additional (and not terribly relevant) thought as well-- due to genetics i do not recall that well, black cats are genetically more likely than any other color to have *strikingly* yellow or green eyes. the extremity of their eyes is somehow tied into their genetic pattern. but blazing yellow eyes in a black face can look more "devilish" than most other combinations.