Okay kiddo, so imagine you and your friends have a secret club and you don't let certain kids join because they're not nice or they don't follow the rules. This is kind of like what blacklisting was in the Soviet Union.
The government in the Soviet Union made a list of people who they didn't like or who they thought might be against them. They would put those people's names on a list and tell other people not to talk to them, work with them or even be friends with them.
It was like a big game of "keep away". The people on the list might have done nothing wrong, but the government didn't like them because of their opinions or ideas.
The government did this to try to control what people thought and said. People were afraid to associate with anyone on the list because they didn't want to get in trouble with the government.
This was not fair and it hurt a lot of innocent people. The people who were blacklisted had a hard time finding jobs or getting help from others because people were afraid to be associated with them.
So, blacklisting was a bad policy because it made people feel scared and alone and it hurt people who didn't deserve it.