What movement sprouted this change in rhetoric? 
By the 1900s, nigger had become a pejorative word in the United States. In its stead, the term colored became the mainstream alternative to negro and its derived terms. After the African-American Civil rights movement, the terms colored and negro gave way to "black". Negro had superseded colored as the most polite word for African Americans at a time when black was considered more offensive. This term was accepted as normal, including by people classified as Negroes, until the later Civil Rights movement in the late 1960s. One well-known example is the identification by Reverend Martin Luther King, Jr. of his own race as "Negro" in his famous speech of 1963, I Have a Dream. During the American Civil Rights movement of the 1950s and 1960s, some African-American leaders in the United States, notably Malcolm X, objected to the word Negro because they associated it with the long history of slavery, segregation, and discrimination that treated African Americans as second-class citizens, or worse. Malcolm X preferred Black to Negro, but later gradually abandoned that as well for Afro-American after leaving the Nation of Islam.