Illuin place-name, name of one of the Lamps of the Valar; apparently incorporating the element luin "blue" (Silm): hence *"all-blue"?
Quenya
luinë
blue
luinë
adjective. blue
Illuin
blue
lúnë
blue
lúnë (stem *lúni-, given the primitive form ¤lugni) adj. "blue" (LUG2, LT1:262; later sources rather give luinë, with pl. form luini_ in Namárië)_. According to VT45:29, lúnë in the Etymologies was changed by Tolkien from lúna.
ninwa
blue
ninwa adj. "blue" (LT1:262)
Menel
heavens
Menel noun "heavens" (Markirya, SA), "the heavens, the firmament" (SD:401), "the apparent dome in the sky" (MR:387). Menel Cemenyë ("k") "Heaven and Earth" (VT47:30). Found in names like Meneldil "Heaven-friend" = astronomer (Appendix A; Letters:386), Meneldur masc. name, "Heaven-servant" (Appendix A, Tar-Menelduras a Númenórean King, UT:210); menelluin "sky-blue", used as noun = "cornflower" (J.R.R. Tolkien: Artist & Illustrator p. 193). Menelmacar "Swordsman of the Sky", the Orion constellation (also called Telumehtar, Appendix E, first footnote); the older name was Menelmacil "Heaven-sword" (WJ:411); Meneltarma "Pillar of Heaven", name of the great central mountain of Númenor (SA:tar, VT42:21).Menelya fifth day of the Eldarin six-day week, dedicated to the heavens (Appendix D) Locative meneldë "in heaven"; abandoned forms menellë, menelzë (VT43:12, 16). Adj. meneldëa "(being) in heaven", evidently based on a locative form meneldë "in heaven"; abandoned forms menelzëa, menellëa, menelessëa (VT43:13, VT44:16; the last of these forms would suggest the locative form #menelessë).
luinë adj. "blue", pl. luini (PE17:66, VT48:23, 24, 28, Nam, RGEO:66). Common Eldarin luini- would also be the stem-form in Quenya (VT48:24). Compare luinincë. Apparently -luin in Illuin, the name of one of the Lamps of the Valar (q.v.), Helluin, name of the star Sirius, and Luinil, name of another blue-shining star (or planet). (SA; Luinil is tentatively identified with Neptune, MR:435). Cf. also menelluin "sky-blue", used as noun = "cornflower" (J.R.R. Tolkien: Artist & Illustrator p. 193).