Several parts combine into one character.

Phonetic-semantic compound: 日 (sun) + 音 (sound). 音 lends its phonetic value (yīn / on) but also a poetic suggestion: "when only sound remains because the sun has gone". Where 明 (bright, earlier batch) is sun + moon together, 暗 (dark) is sun + sound — light replaced by audio. Identical across 繁體 / 新字体 / 简体. Pairs perfectly with 明 (bright).

Mandarin: àn, falling 4th tone. 暗 covers physical darkness, secrecy, and figurative gloom: 黑暗 (hēi'àn, blackness / darkness — common compound), 暗号 (ànhào, secret signal), 暗示 (ànshì, hint / implication), 暗杀 (ànshā, assassination — "dark kill"), 暗中 (ànzhōng, secretly — "in the dark"), 阴暗 (yīn'àn, gloomy / shadowy). The "secret / hidden" extension is more emphasized in Mandarin than in Japanese.

Japanese: on-reading アン (an) for compounds — 暗記 (anki, memorization — "memorize in the dark / by heart"), 暗算 (anzan, mental calculation), 暗号 (angō, code / cipher), 暗示 (anji, hint), 暗黒 (ankoku, darkness), 明暗 (meian, light and shade / bright and dark). The compound 暗記 — to memorize so thoroughly you could recite "in the dark" — is a key learning vocabulary item every Japanese student knows. Kun-reading くら.い (kura.i) is the everyday adjective — 暗い (kurai, dark). Pairs with 明るい (akarui, bright).

In Japanese cultural usage, calling someone 暗い人 (kurai hito, "a dark person") describes a gloomy or introverted personality — a character judgment, not a description of skin or hair. Native speakers use this constantly in casual conversation.

Memory aid: 明 = sun + moon (light); 暗 = sun + sound (light gone, only sound). Direct visual opposites.