The Hollywood blacklist was a massive systematic decision across studios that actually began in the late 1930s and the early 1940s with government investigations into Hollywood, and included people who were merely suspected of having private sympathy to the Communist Party. https://twitter.com/jonathanchait/status/1360240569168121863

Keep in mind that during this timeframe Hollywood, in particular Disney, was interested in Union-busting and they attempted to use an overblown accusation of Communists in Hollywood as an excuse to deny film workers rights, or to bust their unions.

That said, after the Soviet Union switched sides in World War II, this activity died down until after the war when anti-Semitic neofascists like Gerald Smith began to reintroduce the idea to the American populace, referring to "alien minded Russian Jews in Hollywood" in speeches.

Then in 1946 William “Billy”Wilkerson, the publisher of The Hollywood Reporter still bitter about being unable to start his own studio, began to publish names of people he accused of being Communist sympathizers.

Congress, specifically the HUAC, began to investigate the claims. Among the witnesses called were Walt Disney and Ronald Reagan. Both took the opportunity to bash unions, and Reagan’s testimony was one of the factors that led to his divorce from Jane Wyman.

Following further HUAC hearings (the Hollywood Ten) where witnesses refused to answer questions on first amendment grounds, fearing government intervention into their monopolistic hold on the film industry, the studios quickly promised to not hire anyone accused of “red ties”.

The list of blacklisted professionals grew beyond the ten quickly, and was supported not just by the government but also private groups like the American Legion. Additionally a publication was founded explicitly to leak FBI documents from HUAC investigations to grow the blacklist

It’s worth noting, especially to @jonathanchait it seems, that the Hollywood Blacklist was often not based on public statements but rather on private beliefs, personal associations, forced testimony from industry professionals, rumor and innuendo.

One person was blacklisted because his name came up in passing in a HUAC hearing. He wasn’t accused of anything in the hearing, but after his name was mentioned he stopped getting work. That man was Lionel Stander.

If you look at Stander’s filmography you will see that he worked regularly until 1951, when his credited work stops for a full 12 years. It has already slowed because of his role in founding the Screen Actors Guild, mind you...

Stander did ultimately testify before Congress.

Honestly, if you get a chance, read Stander’s HUAC testimony and the play “Have You Now Or Ever” which dramatically presents his testimony—both are excellent.