Let me google that for you...
"Negro" superseded "colored" as the most polite terminology, at a time when "black" was more offensive.[SUP]
[3][/SUP] This usage was accepted as normal, even by people classified as Negroes, until the later
Civil Rights movement in the late 1960s. One well-known example is the identification by
Martin Luther King, Jr. of his own race as 'Negro' in his famous 1963 speech
I Have a Dream.
During the
American Civil Rights movement of the 1950s and 1960s, some
black American leaders in the United States,
notably Malcolm X, objected to the word "Negro" because they associated the word Negro with the long history of slavery, segregation, and discrimination that treated African Americans as second class citizens, or worse.[SUP][4][/SUP] (Malcolm X preferred "Black" to "Negro", but also started using the term "Afro-American" after leaving the
Nation of Islam.)[SUP]
[5][/SUP]