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."
type of poisonous spider (Latrodectus mactans) in U.S. South, 1904, so called from its color and from the female's supposed habit of eating the male after mating (the males seem to get eaten more often before they mate, when they first enter the webs of the females, which have very poor eyesight). Sometimes also known as shoe-button spider. The name black widow is attested earlier (1830s) as a translation of a name of the "scorpion spider" of Central Asia.
In the sense of "person liberally and excessively sympathetic" (especially toward those the speaker or writer deems not to deserve it) is attested by 1951, but said by many to have been popularized with reference to liberals (especially Eleanor Roosevelt) in 1930s by newspaper columnist Westbrook Pegler (1894-1969), though quotations are wanting; bleeding in a figurative sense of "generous" is from late 16c., and the notion of one's heart bleeding as a figure of emotional anguish is from late 14c.; the exact image here may be the "bleeding heart of Jesus."
As a noun, it is recorded from 1809 in the sense "outburst, quarrel;" 1807 as "an explosion." Meaning "enlargement from a photograph" is attested by 1945 (the verbal phrase in this sense is by 1930). Old English had an adjective upablawan "upblown," used of a volcano, etc.