The “Blue Mountains” in western Beleriand and eastern Eriador (S/54, 91), also named Ered Lindon (S/123). This name is a combination of the plural of orod “mountain” and the adjective luin “blue”, which has the same singular and plural forms.
Conceptual Development: In Silmarillion drafts from the 1930s, this name appeared as N. Eredluin “Blue Mountains” (SM/121, LR/267), but at this stage its second element was the lenited plural of N. lhûn, as is clear from its variant name N. Lhúnorodrim (Ety/LUG²).
The mountains in eastern Beleriand and western Eriador named after the region Lindon (S/123). This name is a combination of the plural of orod “mountain” with that region name.
Conceptual Development: In Silmarillion drafts from the 1930s, this name first appeared as N. Eredlindon, but it was first translated “Blue Mountains” (LR/126, LR/260). The second element of this name was originally derived from N. glinn “(pale) blue”, but in The Etymologies Tolkien rejected this word (Ety/GLINDI, EtyAC/GLINDI). He then revised this name’s derivation to the one given above, though at this stage, Lindon was an Ilkorin name (Ety/LIN², ÓROT). Despite this change, Tolkien still referred to them as the “Blue Mountains” in The Lord of the Rings (LotR/43), but with a new Sindarin name Ered Luin.