There’s a few different theories as to why it's called Black Friday. One comes from Philadelphia in America. The local Police are said to have coined the term while planning ahead for all the heavy traffic that would arrive as people came to shop the Black Friday deals in-store.
Another popular theory is that the name comes from bookkeeping. When a company is in debt, their accounts are in the red. When a company is in profit, they're in the black. The demand for Black Friday deals could be so big that many retail stores would finally see their yearly accounts go from the red and into the black.