The Blue Mountains or Ered Luin, also known as Ered Lindon, was the mountain range at the far west of Eriador.
During the First Age, the Blue Mountains was an unbroken line separating Eriador from Beleriand. Seven rivers arose in it on the western side, and the land these rivers flowed through was known as Ossiriand, which was later also as Lindon, therefore the mountains were sometimes referred to as the Ered Lindon. The dwarven cities of Belegost and Nogrod were located in this range.
The mountain range was broken during the war of the Valar against Morgoth, and at the middle of the range the sea broke through, creating a new terminus for the river Lhûn. In the middle of the gap, where the Lhûn met the sea, the Grey Havens of Mithlond of the Elven Kingdom of Lindon were built. When seen as the border of Lindon the Ered Luin were called Ered Lindon.
During the third age, its forests, besides that of Mirkwood, were the largest in Middle-earth.
During the creation of Arda, the Blue Mountains were meant to line up directly with the Grey Mountains of the southlands, forming the western wall of Middle-earth, just like the Red Mountains (or Orocarni) and Yellow Mountains formed the eastern wall. The range of the Blue Mountains was originally connected with the Red Mountains by the Ered Engrin or Iron Mountains. But this symmetry of Arda was broken during the wars before the Years of the Lamps.
The Blue Mountains, the Iron Hills, and the Lonely Mountain (Erebor) were the main areas of Dwarf colonization by the late Third Age. During the 4000+ years following the cataclysmic sundering and catastrophic downfall of Beleriand, when the mighty Dwarf fortress-cities of Nogrod and Belegost were inundated by the sea, it is possible that the Dwarves rebuilt those cities to their former splendor.
The Dwarves would have plundered the mines of the Blue Mountains for the hoard of resources it held within the depths of darkness at its roots.
Before setting off in their quest to the Lonely Mountain, Thorin Oakenshield and his 12 other companions (as well as other dwarves of Erebor) resided here in the Blue Mountains after being rendered homeless by the dragon Smaug.
It is shown in several maps made of Middle-earth that great forests flanked both sides of the Blue Mountains, even after the gargantuan slaughter of Eriador's great forests by the men of Númenor and their incessant appetite for lumber. These forests, alongside the fertile coastal plain, would prove to be hotspots for Elvish migration and inhabitation.
It is stated at the very beginning of The Hobbit that the High Elves lived within the vales and forests of the Blue Mountains, presumably sharing the land with the Dwarves. To this number would be added Sindar, Wood-elves, and maybe even a few Avari.
The influx of new Elves skyrocketed after the end of the First Age, the Second, and the Third, probably due to these particular Elves desiring to escape the woes and predicaments of Middle-earth and begin a life anew in the Undying Lands.
Although the Blue Mountains were little described (owing to their location on the fringes of all of Tolkien's tales), we can assume a bit of their geology. The outer layers of the mountains seem to have been underlaid by metamorphic rocks formed in contact with numerous igneous intrusions. This environment is often necessary to produce veins of ore such as those mined by the dwarves from the beginning of time. This environment produced the trove of Gold, Silver, iron, gems, and other materials that made the Dwarf kingdoms incredibly wealthy.
The Blue mountains were shorter than the Misty Mountains.
The Blue Mountains are also known as Ered Luin, which is from the Sindarin words Ered ('Mountains') and Luin ('Blue'). Other names for the Blue mountains are Ered Hithui, which means 'misty mountains' in Sindarin, Hithaeglir, which is from the words Hith (mist), aegas (mountain peak) and lîr (range) in sindarin, and Ered Lindon. 
Portrayal in AdaptationsEdit
Central Ered Luin is amongst the regions fleshed out in The Lord of the Rings Online. Significant areas surrounding Dwarven and Elven settlements are available for exploration and adventuring. The Blue Mountains are only in the campaign in The Lord of the Rings: The Battle for Middle-earth II but may not in a skirmish. It also has it's own biome in The Lord of the Rings Mod: Bringing Middle-earth to Minecraft.
|Mountain Ranges of Arda|
Ash Mountains | Echoriath | Ephel Dúath | Ered Gorgoroth | Blue Mountains | Ered Lómin | Grey Mountains | Ered Wethrin | Iron Hills | Iron Mountains | Misty Mountains | Mountains of Angmar | Mountains of Mirkwood | Mountains of Mithrim | Orocarni | Pelóri | Wall of the Sun | White Mountains | Yellow mountains
|Dwarven Realms of Middle-earth throughout the Ages|
|Years of the Trees & First Age:||Bar-en-Nibin-Noeg | Belegost | Khazad-dûm | Mount Gundabad | Nogrod | Nulukkizdîn | Orocarni|
|Second Age:||Khazad-dûm | Mount Gundabad|
|Third Age:||Grey Mountains | Iron Hills | Khazad-dûm | Lonely Mountain | Northern Blue Mountains|
|Fourth Age:||Glittering Caves | Khazad-dûm | Lonely Mountain|