Obsidian black
A black so dense it reflects — the colour of cooled fire and of edges that have been polished.
Obsidian black in a dream is the dream's image of a black so dense that it reflects. Obsidian is volcanic glass; it was once liquid fire and is now a hard, polished mirror that can be flaked into a blade. The dream brings the colour when something in your life — an anger, a grief, a piece of history — has cooled into something hard and useful. An obsidian surface in which you can see your face is the dream telling you the cooled material is now a mirror you can consult. An obsidian edge in your hand is the dream noting that the cooled thing has been shaped into a weapon, and the use to which you put it now matters.