There are a couple of ways to achieve a traditional black barn finish. Firstly you can use a black wood preservative such as Ronseal Total Wood Preservative, Ronseal Shed and Fence Preserver or Cuprinol Exterior Wood Preserver to colour and protect the barn from mould, algae and wood boring insects. These products can be used as a standalone treatment or overcoated with a clear exterior wood oil or decking oil to give the wood additional protection from weathering. For this, consider Barrettine Log Cabin Treatment.