Elbrus, the Binding Blade, is a Legendary Artifact - Equipment that transforms into Withengar Unbound, a Black Legendary Demon. Can I use Elbrus/Withengar as a Commander, and if I can, is my deck colorless or black?
Here's an official ruling from Sheldon Mennery, who is the authoritative source for Commander rulings:
[O] Elbrus is not playable as a General, and its color identity is Black.
Elbrus, the Binding Blade is not a Legendary Creature, so it cannot be your Commander. It doesn't matter that it could become a creature. In fact, I'd be surprise if there is a card that couldn't become a creature.
711.2a In every zone other than the battlefield, and also on the battlefield with its front face up, a double-faced card has only the characteristics of the front face.