The number and date "13" is taboo for religious reasons. According to the Bible, Jesus called 12 disciples to preach throughout Judea. Later, he was hated by the Jewish authorities, arrested and sent to Pilate, the governor of the Roman Empire in Judea, and was crucified. Since then, Christians have regarded this number and date as taboo. Russians never invite 13 guests, and avoid 13 every month when they get married. Families generally don't hold commemorative activities on 13.
Black Friday doesn't mean unlucky. Because the Friday before Christmas is the climax of American consumption, almost every store is profitable, and the traditional American bookkeeping method is to use red ink to represent losses (the origin of deficits) and black ink to represent profits, so Friday is called Black Friday.
However, because the day when Jesus was crucified happened to be Friday, Friday is also taboo in the hearts of westerners!