Middle English derk, later dark, from Old English deorc "without light, lacking light or brightness (especially at night), obscure, gloomy;" figuratively "sad, cheerless; sinister, wicked," from Proto-Germanic *derkaz (source also of Old High German tarchanjan "to hide, conceal"), which is of uncertain etymology. For vowel change, see marsh.
Application to colors, "not radiating or reflecting much light," is from late 14c. Of complexion, "not fair," from early 14c. Figurative sense of "obscure, not easily understood" is from early 13c.; that of "sullen, sad" is from 1590s. Meaning "concealed, secret" is from late 14c. Dark Continent "Africa" (1828) combines several figurative senses (earliest references are in missionary publications). Theater slang for "closed" is from 1916.
Dark Ages "benighted time in history, period of ignorance" is attested by 1739; the specific focus on the centuries of the early Middle Ages in Europe, from the fall of Rome to the revival of secular literature, is from 1830s, from dark in a sense of "characterized by ignorance, backward in learning, void of intellectual light" (late 14c.).
Dark horse "competitor for honors or office about whom nothing certain is known, or whose identity is at first concealed," especially, in U.S., politics, "one who is unexpectedly brought forward as a candidate in a convention," 1842, is an image from horse racing, of horses whose performances or capabilities are not generally known, in which dark is used in its figurative sense of "unknown."
Moonraker is called a "dark horse"; that is neither his sire nor dam is known. ["Pierce Egan's Book of Sports," London, 1832]
early 13c., derk, "absence of light, night-time," from dark (adj.). Figurative in the dark "in a state of ignorance" is from 1670s; earlier it meant "in secrecy, in concealment" (late 14c.).
"period between ancient and modern times" (formerly roughly 500-1500 C.E., now more usually 1000-1500), attested from 1610s, translating Latin medium aevum (compare German mittelalter, French moyen âge).
"dark-colored, of brown tinged with gray," 1660s, from Latin fuscus "dark, swarthy, dark-skinned" (see dusk). Earlier as fusc, fusk (1560s).
Old English dunn "dingy brown; dark-colored," perhaps from Celtic (compare Old Irish donn "dark;" Gaelic donn "dull; dark brown; dark;" Welsh dwnn "brownish"), from PIE *donnos, *dusnos "dark." As a noun, "dun color," 1560s; as "a dun horse" from late 14c. The "horse" meaning is that the figurative expression dun is in the mire "things are at a standstill or deadlocked," which occurs in both Chaucer and Shakespeare. Dun also is likely the origin of the surnames Dunn, Dunne, Donne, Dunning, etc.
"full of darkness," late 15c., from Old French tenebros "dark, gloomy" (11c., Modern French ténébreux), from Latin tenebrosus "dark," from tenebrae "darkness," dissimilated from earlier *temebrai, from Proto-Italic *temasra- "darkness," from PIE root *temsro- "dark" (adj.), for which see temerity. Related: Tenebrosity.
"extreme venturesomeness, rashness, recklessness," late 14c., from Latin temeritatem (nominative temeritas) "blind chance, accident; rashness, indiscretion, foolhardiness," from temere "by chance, at random; indiscreetly, rashly, recklessly;" probably, etymologically, "blindly," from PIE root *temsro- "dark" (adj.), source also of Sanskrit tamisra- "dark night," tamsrah "dark;" Avestan temah "darkness;" Middle Persian tar "darkness," tarig "dark;" Lithuanian tamsa "darkness," tamsus "dark;" Old Church Slavonic tima "darkness;" Old High German dinstar "dark," demar "twilight;" Old Irish temel "darkness."
The connecting notion would be "blindly, in darkness," hence "without foreseeing." Compare Latin tenebrio "dishonest person," apparently "person who operates in darkness" (see tenebrous).