Just like barracks:dark barracks and spell factory:dark spell factory, the use of elixir and dark elixir are seperated. With dark laboratory, you can upgrade dark troops and dark spells and so an upgrade also in the laboratory. It's like an idea of two ugrading troop building. Also the upgrade of hero will be in the dark laboratory. But heroes also require a little bit of gold and dark elixir to be upgraded. Just for clashers to be a little bit busier in the game.
