This was known as Sarlluin in the Legacy version, but is now known as Luinon.
Luinon (Sindarin: Blue Stone) is the light-blue stone found exclusively in the Blue Mountains. There, it is present in a layer overground, covering regular stone, and in randomly generated deposits underground.
It is similar to caranon, which is found in the Red Mountains. While Tolkien himself never mentioned a type of rock specific to the Blue Mountains, this blue stone has been included to explain their name.
Luinon must be mined with a pickaxe. It can be alloyed with iron ingots or iron ore in a Dwarven forge to forge Blue Dwarven Steel. Luinon can also be used to craft Luinon bricks, stairs, slabs, pillars, walls, pressure plates, buttons, and a smooth variety and slabs on a vanilla crafting table. A block of Luinon can be smelted into a Scorched Stone.