proverbial for "something extremely rare or non-existent" (late 14c.) is from Juvenal ["Sat." vi. 164], but the real thing turned up 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."
In reference to dark eyes, often as a mark of beauty, from 1660s. Black-eyed is from 1590s of women, of peas from 1728. The black-eyed Susan as a flower (various species) so called from 1881, for its appearance. It also was the title of a poem by John Gay (1685-1732), which led to a popular mid-19c. British stage play of the same name.
All in the Downs the fleet was moored,
The streamers waving in the wind,
When black-eyed Susan came aboard,
"Oh! where shall I my true love find?
Tell me, ye jovial sailors, tell me true,
If my sweet William sails among the crew?"
1961, "comedy that deals in themes and subjects usually regarded as serious or taboo," from black (adj.), in a figurative sense of "morbid," + comedy. Compare French pièce noire, also comédie noire "macabre or farcical rendering of a violent or tragic theme" (1958, perhaps the inspiration for the English term) and 19th-century gallows-humor. In a racial sense, from 1921.
by 1792, in the figurative sense of "person of bad character; member of some group guilty of offensive conduct that does little credit to the flock, family, or community to which he belongs," supposedly because a real black sheep (there was proverbially one in every flock) had wool that could not be dyed and thus was of less worth. But one black sheep in a flock was considered good luck by shepherds in Sussex, Somerset, Kent, Derbyshire. First known publication of Baa Baa Black Sheep nursery rhyme is in "Tommy Thumb's Pretty Song Book" (c. 1744).