From the wiki:

According to Channing’s memoirs, when she left home to attend Bennington College in Vermont, her mother informed her that her father, a journalist whom she had believed was born in Rhode Island, was in fact born in Augusta, Georgia to a German American father and an African American mother. According to Channing’s account, her mother didn’t want her to be surprised “if she had a black baby”. Channing kept her heritage secret so she would not be typecast on Broadway and in Hollywood, ultimately revealing it only in her autobiography, Just Lucky I Guess, published in 2002 when she was 81 years old. Channing’s autobiography, although containing a photograph of her mother, displays none of her father or her son.