àn is bound — it always appears with a partner. Two compound families. (1) Visual dimness:
黯淡 (dim, lustreless — of light, colour, prospects),
昏暗 (dim, murky — note this is the synonym
暗 not
黯). (2) Emotional gloom:
黯然 (dejected, downcast — adverbial; the workhorse compound),
黯然失色 (pale by comparison, eclipsed),
黯然神伤 (heart-stricken, low-spirited),
黯然销魂 (dispirited to the depths of one's soul — from Jiang Yan's
《别赋》).