This root and ones like it were the basis for shadowy things throughout Tolkien’s life, but went through a number of minor conceptual shifts. The earliest appearance of this root was as ᴱ√LUVU in the Qenya Lexicon of the 1910s, with derivatives like ᴱQ. lumbo “dark lowering cloud” and ᴱQ. lūre “dark weather” (QL/57). In the Poetic and Mythological Words of Eldarissa written afterwards, Tolkien gave the root as ᴱ√LUB with a similar set of derivatives (PME/57); phonological developments in both Early Qenya and Gnomish make it very difficult to distinguish ancient voiced stops [b] from voiced spirants [β]. Sign of this root can also be seen in Gnomish words G. lum or glum “cloud”, G. lumbri “foul weather”, and G. luv- “hang, lower, of clouds” (GL/55).
The derivatives of this root in the 1910s seem to connect more specifically to dark weather, but in The Etymologies of the 1930s the root reappeared as ᴹ√LUM with derivatives having to do mainly with shadow, such as ᴹQ. lumbe “gloom, shadow” and N. lhum “shade” (Ety/LUM). These in turn served as the basis for N. Hithlum and ᴹQ. †Hísilumbe >> ᴹQ. Hisilóme interpreted in this period as “Mist-and-Dusk” (LR/406). In earlier writing the second element of ᴱQ. Hisilóme “Misty-gloom” was derived from ᴱ√LOMO (QL/55), whereas in 1964 notes Hithlum was designated “North Sindarin” and given a new etymology as a direct loan from its Quenya equivalent, and thus no longer connected to √LUM (PE17/133).
The last appearance of the root in currently published material was as √LUM or √LUB “shadow, darkness” with derivatives Q. lumbo “dark, shade” and Q. lumbule “shadow” (PE17/168). Q. Luvailin “Shadowmere” (RC/217) is probably related and must derived from √LUB. For purposes of Neo-Eldarin, I would assume that √LUB is the form as the root, as the various lumb- form can likewise be derived from √LUB by way of strengthened ✱lu(m)b-.