In 1961 Martin Luther King would have been using the word. Later than that in fact. He used it many times, in his "I Have a Dream" speech, for example.
Interesting that in that speech, he uses Negro as a formal noun over 13 times but “black” only as an adjective. He had the rhetorical sense to know that “little black children” and “little white children” sounds far better than “little Negro children” and “little Caucasian children.”