While adoption rates in general are low for cats in animal shelters, black cats have the worst of it. They are the least likely color of cat to be adopted according to a 2002 study. Approximately 20% of cats in animal shelters get a adopted to a good home (the other 80% likely get euthanized). Of those cats, black cats are only half as likely as a tabby cats to get adopted, and ? less likely that white cats.
In Western culture, black cats are considered a bad omen. This was likely exacerbated by "The Black Cat", a short story from Edgar Allen Poe in which a black cat drives his owner crazy and turns him into a murderer. Some animal shelters also suspend the adoption of black cats in October to make sure the adopters aren't trying to use them as a Halloween party gimmick or an animal sacrifice.
Strangely enough, black cats came about from a genetic mutation that keeps their hair dark and prevents them from having stripes. So while black cats may not cause bad luck, black cats in shelters have certainly had bad luck of their own!