If you want to make an authentic Dry Irish Stout, then you need to use roasted barley. It is made from unmalted barley, making it distinct from Black Malt which is made by roasting malted barley. This creates a dry, roasted flavor with hints of coffee, and produces a beer with with a creamy white to off white head. It gives black color with ruby highlights and the distinct, dry, roasted bitter flavor characteristic of Guinness.
It can also be used in very small amounts for color adjustment with little flavor contribution. 500-600°L