proverbial for "something extremely rare or non-existent," late 14c., from Juvenal ["Sat." vi. 164], but the real thing turned up later in Australia (Chenopsis atratus).
"Do you say no worthy wife is to be found among all these crowds?" Well, let her be handsome, charming, rich and fertile; let her have ancient ancestors ranged about her halls; let her be more chaste than all the dishevelled Sabine maidens who stopped the war—a prodigy as rare upon the earth as a black swan! yet who could endure a wife that possessed all perfections? I would rather have a Venusian wench for my wife than you, O Cornelia, mother of the Gracchi, if, with all your virtues, you bring me a haughty brow, and reckon up Triumphs as part of your marriage portion. [Juvenal]
Blue dahlia also was used 19c. for "something rare and unheard of."
"right of each citizen to liberty, equality, etc.," 1721, American English, from civil in the sense "pertaining to the citizen in his relations to the organized commonwealth or to his fellow citizens." Specifically of black U.S. citizens from 1866, in reference to the Civil Rights Bill, an act of Congress which conferred citizenship upon all persons born in the United States, not subjects of other powers, "of every race and color, without regard to any previous condition of slavery." Civil Rights Movement in reference to the drive for racial equality that began in U.S. in mid-1950s is attested by 1963.
Rarely are we met with a challenge, not to our growth or abundance, or our welfare or our security, but rather to the values and the purposes and the meaning of our beloved nation. The issue of equal rights for American Negroes is such an issue. And should we defeat every enemy, and should we double our wealth and conquer the stars, and still be unequal to this issue, then we will have failed as a people and as a nation. [Lyndon Johnson, speech introducing Voting Rights Act, March 15, 1965]