A black list is a list of people who have been banned from some particular activity or organization. In United States history, the term “blacklist” is most commonly associated with the red scare of the 1950s.
In the 1950s, there was a strong fear of communism in the United States. There were many who feared that the communists would subvert Americans and cause them to become communist themselves. For this reason, there were various attempts to expose alleged communists to prevent them from corrupting other Americans.
The most famous blacklist during this time was a blacklist that applied to people who had been in the movie industry. Those who were allegedly communists were put on this blacklist and were prevented from working in Hollywood. Thus, it was a blacklist because they were banned (unofficially) from working in the movie industry.