The most notable uses of this root were as the basis for the name Q. Melkor and (sometimes) the adjective S. beleg “great”. This root first appeared as ᴱ√mbelek, belek or melek in the Gnomish Lexicon of the 1910s, with Gnomish name G. Belcha vs. Qenya name ᴱQ. Melko; its other derivatives indicate the meaning of the root was “flame” (GL/20). The Qenya noun ᴱQ. velka “flame” indicates that unstrengthened ᴱ√belek was used along with strengthened ᴱ√mbelek, but there are no obvious derivatives of ᴱ√melek* in this period.
There is no evidence of this root in The Etymologies of the 1930s; in this period ᴹQ. Melko was derived from ᴹ√MIL(IK) “greed, lust” (Ety/MIL-IK). The root appears a number of times in Tolkien’s later writings, always as the basis for Q. Melkor. The root melk- was mention in notes associated with the essay Athrabeth Finrod ah Andreth from around 1959, where Tolkien said it “means ‘power’ as force and strength” (MR/350). The root appears as √MELEK “great, mighty, powerful, strong” (rejected) or mbelek “large, great” in notes from the late 1950s or early 1960s, connected to both S. beleg and Q. Melkor (PE17/115). The root √MBELEK is implied by Tolkien’s etymology of Q. Melkor in the Quendi and Eldar essay from 1959-60, being derived from ✶Mbelekōre* “He who arises in Might” (WJ/402).
It seems Tolkien vacillated on whether Q. Melkor and S. beleg were related. The root may have been √MELEK, unrelated to √BEL. Alternately, it may have been √MBELEK, but the various mutations of S. beleg “great” show no signs of primitive initial mb-. Thus, it seems the strengthening to mb- either occurred only in Quenya, or it enhanced the meaning of the root from “large, great” to “powerful, mighty”.
Neo-Eldarin: For purposes of Neo-Eldarin, I prefer the last of these theories.