chiaroscuro n.

c. 1300, "giving light, shining, luminous;" also "not turbid; transparent, allowing light to pass through; free from impurities; morally pure, guiltless, innocent;" of colors, "bright, pure;" of weather or the sky or sea, "not stormy; mild, fair, not overcast, fully light, free from darkness or clouds;" of the eyes or vision, "clear, keen;" of the voice or sound, "plainly audible, distinct, resonant;" of the mind, "keen-witted, perspicacious;" of words or speech, "readily understood, manifest to the mind, lucid" (an Old English word for this was sweotol "distinct, clear, evident"); of land, "cleared, leveled;" from Old French cler "clear" (of sight and hearing), "light, bright, shining; sparse" (12c., Modern French clair), from Latin clarus "clear, loud," of sounds; figuratively "manifest, plain, evident," in transferred use, of sights, "bright, distinct;" also "illustrious, famous, glorious" (source of Italian chiaro, Spanish claro), from PIE *kle-ro-, from root *kele- (2) "to shout."

The prehistoric sense evolution to light and color involves an identification of the spreading of sound and the spreading of light (compare English loud, used of colors; German hell "clear, bright, shining," of pitch, "distinct, ringing, high").

Also in Middle English "beautiful, magnificent, excellent" (c. 1300); of possession or title, "unrestricted, unconditional, absolute," early 15c. Of complexion, from c. 1300. Sense of "free from encumbrance," later largely nautical, developed c. 1500. Meaning "obvious to the senses" is from 1835. Clear-sighted is from 1580s (clear-eyed is from 1520s); clear-headed is from 1709. For coast is clear see clear (v.).

c. 1400, "dark," figuratively "morally unenlightened; gloomy," from Old French obscur, oscur "dark, clouded, gloomy; dim, not clear" (12c.) and directly from Latin obscurus "dark, dusky, shady," figuratively "unknown; unintelligible; hard to discern; from insignificant ancestors," from ob "over" (see ob-) + -scurus "covered," from PIE root *(s)keu- "to cover, conceal." Related: Obscurely.

The figurative sense of "not readily understood, not manifest to the mind or understanding" is from early 15c.; of persons, "not illustrious or noted, unknown to fame," 1540s. The more literal sense of "indistinct, without clearness of form or outline, hardly perceptible, not capable of being clearly seen through lack of light" is attested in English from 1590s.

In regard to the meaning of something said or written, obscure is general, being founded upon the figure of light which is insufficient to enable one to see with any clearness; this figure is still felt in all the uses of the word. [Century Dictionary, 1895]