A word for “darkness” or “blackness” appear in the Namárië, a noun form of morna “black, dark” (LotR/377; PE17/73).
Quenya
mornië
darkness
mornië
noun. darkness, blackness
mórë
blackness, dark, night, darkness
mórë noun "blackness, dark, night, darkness" (MOR, MC:214), also given with a short vowel:morë "dark, darkness" (Letters:282). If this is the initial element of Morinehtar "Darkness-slayer" (PM:384, 385), it would seem to have the stem-form mori-, though mori- is normally the adjective "dark, black" (see below).
-ië
openness
-ië (2) abstract ending, often used to derive abstracts from adjectives, e.g. látië "openness" vs. láta "open", mornië "darkness" vs. morna "black, dark", vanië (for *vanyië) "beauty" vs. vanya "fair".
mor
darkness
mor noun "darkness" (Letters:308; probably just an Elvish "element" rather than a complete word; Namárië has mornië for "darkness")
lúmë
noun. darkness
A noun in the 1960s versions of the Markirya glossed “darkness” (MC/222), perhaps derived from a root √DU as suggested by David Salo in a post to the Elfling mailing list in 2012 (Elfling/362.96).
Neo-Quenya: I’d generally use Q. huinë for “darkness” in Neo-Quenya, but that word is more for total darkness, whereas lúmë might be a less severe form of darkness, a variant of Q. lómë “night, dusk”.
látie
noun. openness
látië
openness
látië noun "openness" (VT39:23)
lúmë
darkness
lúmë (2) noun "darkness" (one wonders if Tolkien confused lúmë "time, hour" and lómë "night") (Markirya)
mornië noun "darkness" (Nam, RGEO:67), "dark, blackness" (PE17:73). Early "Qenya" also has Mornië "Black Grief", "the black ship that plies between Mandos and Erumáni" (LT1:261). This is probably a compound of mor- "black" and nië "tear".