Why are black cats unluky?

Answer 1

Black cats are unlucky only because people believe they are unlucky.

Cats have always been somewhat mistrusted, due to their solitary nature, their silence and evasiveness, and the odd way their eyes reflect even small amounts of light. Because a cat will associate with humans, but not, apparently, accept a human as its master, many societies have a belief that cats cannot be trusted. This has led to an association of cats in general with evil, notably witches and the devil. Black cats, being effectively invisible at night, were mistrusted more than other colors. This mistrust evolved over time into a belief that black cats brought bad luck.

Due to what is called the "placebo effect", and "magical thinking", people who believe a black cat is bad luck, who encounter a black cat, will start to experience bad luck. If the exact same events happen to a person who has not encountered a black cat, or who does not believe a black cat is bad luck, they will be seen as happenstance. This, of course, re-inforces the belief.