Superstitions surrounding black cats can vary from culture to culture, but black cats have positive associations in Celtic nations and Japanese folklore. Black cats were sacred in Celtic mythology, while in Scottish tradition they say that the arrival of a black cat in a new home signifies prosperity. Welsh tradition states that a black cat brings good health.

However, both the Gaels and the Celtic Britons had traditions of wild and sometimes malevolent black cats. In Scottish mythology, a fairy known as Cat sìth takes the form of a black cat, while in Welsh mythology, the monstrous Cath Palug grew from a black kitten. In England, as in other Germanic cultures, some areas would associate black cats with witches and bad luck. The combination of positive and negative associations in Britain may have given rise to the later belief that black cats were omens of both good and bad luck.

A tradition says that if a black cat walks towards someone, it is said to bring good fortune, but if it walks away, it takes the good luck. This tradition was reversed at sea, where 18th century pirates came to believe that a black cat would bring bad luck if it walked towards someone and good luck if it walked away from someone. It was also believed that if a black cat gets on a ship and then gets off, the ship is doomed to sink on its next voyage. Also, it is believed that a lady who owns a black cat will have many suitors.

Black cats are often a symbol of Halloween or witchcraft. In most Western cultures, black cats have generally been regarded as a symbol of evil omens, specifically they are suspected of being the familiars of witches or, in reality, the shape-shifting witches themselves. Most of Europe considers the black cat a symbol of bad luck, especially if one crosses the road in front of a person, which is believed to be an omen of misfortune and death. In Germany, some believe that black cats crossing a person's path from right to left is a bad omen, but from left to right, the cat is bestowing favorable times.

The black cat in folklore has been able to change into human form to act as a spy or messenger for witches or demons. When the Pilgrims arrived at Plymouth Rock, they brought with them a devout faith in the Bible. They also brought a growing suspicion of anything considered Satanic and deeply suspicious of other Christians, including those of the Catholic, Quaker, Anglican, and Baptist denominations. Pilgrims viewed the black cat as a companion or familiar to witches, who were said to "use black cats as an integral part of their craft." These superstitions led people to kill black cats. There is no evidence from England of regular large-scale massacres of "satanic" cats, or burning them at stake in midsummer, as was sometimes the case elsewhere in Europe. Today, many Westerners, including Christian clergy, keep black cats as pets, and very few people attach superstitions to them.

In contrast, the supernatural powers attributed to black cats were sometimes seen as positive; for example, sailors considering a "ship's jack" would want a black one because it would bring good luck. Sometimes fishermen's wives also kept black cats at home, hoping they could use their influence to protect their husbands at sea. In the Chiloé folklore of southern Chile, black cats are an important element that is needed when hunting for the treasure of the carbunclo. In the early days of television in the United States, many stations located on VHF channel 13 used a black cat as a mascot for the amusement of being located on an "unlucky" channel number.

October 27 has been designated as "National Black Cat Day" by Cats Protection in the United Kingdom of Great Britain and Northern Ireland. In the United States "National Black Cat Appreciation Day" is on August 17 to celebrate the virtues of black cats and to encourage people to adopt an unwanted black cat. Cats Protection's own figures suggest that black cats are more difficult for them to find a new home than other colors.