Opposite words for bright:
somber, lunatic, dreary, doubtful, desolate, unwise, unlucky, pitch-dark, blank, inauspicious, soft, balmy, preposterous, raw, lacklustre, dimmed, depressing, negative, cheerless, miserable, moronic, dense, sluggish, saturnine, colorless, unfortunate, unencouraging, unpolished, sinister, pessimistic, sunless, insignificant, absurd, severe, boneheaded, matte, unbright, dull, half-baked, darksome, catastrophic, thickheaded, asinine, silly, brainless, dumb, forlorn, lethargic, disheartened, brokenhearted, daffy, loony, opaque, scowling, pale, rough, dark, darkish, ignorant, crackpot, dusk, subdued, listless, shady, cuckoo, lamebrain, glowering, foreboding, feebleminded, birdbrained, illiterate, portentous, tomfool, thick-witted, discouraging, lowbrow, low-spirited, darkened, misty, minor, uncelebrated, dorky, foolish, mirthless, bungling, unthinking, nonintellectual, senseless, wretched, knuckleheaded, ruinous, unlikely, witless, murky, mediocre, empty-headed, fatal, fatuous, screwball, fool, dubious, crazy, unemotional, simpleminded, untaught, minatory, kooky, sorrowful, dusky, cloudy, dopey, overcast, gray, lightless, unreverberant, unsmart, lightproof, slow-witted, featherheaded, soupy, blue, uneducated, menacing, lunkheaded, hopeless, disastrous, unlit, blustering, insane, droopy, comfortless, mat, airheaded, vacuous, imbecile, tempestuous, melancholy, blustery, stupid, obscured, crestfallen, obscure, weak-minded, chuckleheaded, dispiriting, matted, pitch-black, daft, downhearted, unintelligent, shadowy, downcast, morose, obtuse, thick, black, unpropitious, drab, doomy, wacky, harebrained, inclement, harsh, softheaded, ill-boding, breezy, dim, gormless, lowering, shadowlike, unhappy, uninformed, dejected, unsatisfied, unimportant, grim, palish, hazy, brain-dead, dirty, cockeyed, threatening, dismal, ill, slow, disconsolate, unintellectual, idiotic, low, torpid, zany, unacademic, mad, average, depressed, unsung, dim-witted, thick-headed, gusty, downbeat, bubbleheaded, stoic, nutty, baleful, pinheaded, sad, foggy, foul, bleak, nonresonant, dire, mindless, dour, nasty, glum, gloomy, glaring, squally, down, stormy, dotty, rainy, darkening, unpromising, funereal, despondent, desperate, sullen, clumsy, inglorious, darkling, hangdog, unfavorable, uncertain, direful, half-witted, unbrilliant, inferior, tenebrous, turbulent, heavyhearted, lustreless, flat, doltish, colourless, sappy, disheartening, ominous, frowning, awkward, sulky, blackened, lackluster, boobish, nonsensical, dunderheaded, dippy, matt, bearish, leaden, simple, inconsolable, futureless, lusterless, joyless, calamitous.
Usage samples:
  1. The key often used is always bright.
    "Young's Demonstrative Translation of Scientific Secrets", Daniel Young.
  2. The dark woman's eyes were bright; she spoke with great pride.
    "A Canadian Bankclerk", J. P. Buschlen.
  3. A bright light broke on Peterkin.
    "The Last Shot", Frederick Palmer.
  4. Old Man Bright died.
    "The Killer", Stewart Edward White.