This is what it says about Charles Korvin (Carlos in "Mama Loves Mambo") on IMDb:
Blacklisted in Hollywood ca. 1951 when he refused to testify before the U.S. House of Representatives Committee on Un-American Activities. He remained blacklisted for about ten years.

I believe 1951 was the same year Pert Kelton was blacklisted, but how come CBS wouldn't allow her to continue playing Alice, but Mr. Korvin was allowed to appear in "Mama Loves Mambo" on the same network in 1956?

This is what it says about Charles Korvin (Carlos in "Mama Loves Mambo") on IMDb:

Blacklisted in Hollywood ca. 1951 when he refused to testify before the U.S. House of Representatives Committee on Un-American Activities. He remained blacklisted for about ten years.
I believe 1951 was the same year Pert Kelton was blacklisted, but how come CBS wouldn't allow her to continue playing Alice, but Mr. Korvin was allowed to appear in "Mama Loves Mambo" on the same network in 1956?
Interesting question, innit? However, keep in mind that guests and other supporting actors (i.e. George Petrie, Frank Marth) were never credited in the closing credits of the 39 episodes. However, a few years after the "Mama Loves Mambo" episode, Mr. Korvin played "The Eagle" in the first season of the 1950's TV series Zorro.

But it should also be noted that the acerbic radio comedian Henry Morgan was briefly blacklisted before landing the panelist spot on I've Got a Secret which he joined in 1952, a few episodes into its run, and remained on the panel until its cancellation in 1967.

^^It's strange how exceptions were made in certain cases, while Pert Kelton was unable to continue playing (what would be) her best-remembered role. You make a good point about the guests not being credited though...maybe CBS never looked into it when Mr. Korvin appeared on the show. Pert was able to return to the show in the 60's to play Alice's mother though, so the blacklist couldn't have been in effect anymore, or else I'm sure they wouldn't have let her appear.

^^It's strange how exceptions were made in certain cases, while Pert Kelton was unable to continue playing (what would be) her best-remembered role. You make a good point about the guests not being credited though...maybe CBS never looked into it when Mr. Korvin appeared on the show. Pert was able to return to the show in the 60's to play Alice's mother though, so the blacklist couldn't have been in effect anymore, or else I'm sure they wouldn't have let her appear.
By the time of Ms. Kelton's 1967 appearance, the blacklist had long ago been broken. Remember John Henry Faulk's successful libel suit against Aware, Inc. (as documented in his Fear on Trial book)? She was also in The Music Man and an episode of The Twilight Zone.

By the time of Ms. Kelton's 1967 appearance, the blacklist had long ago been broken. Remember John Henry Faulk's successful libel suit against Aware, Inc. (as documented in his Fear on Trial book)? She was also in The Music Man and an episode of The Twilight Zone.
I'm not really too familiar with the whole story of the blacklist, so thanks for clearing that up for me. The information I posted about Mr. Korvin said that he was blacklisted for almost 10 years, so I assumed it had to have been broken sometime between then and Pert's appearance on TJGS in 1967.