The black stains on Redwood are caused by the very thing that makes Redwood resistant to bugs. Tannic Acid is contained the lumber. When it comes in contact with any metal, a black stain will form. In addition , over time, Redwood will trun a grayish color because of Tannic Acid. There are many products on the market that will take off the stain. Oxalic Acid, mixed with water will work. Pressure wahing will work although one has to be careful to not raise the grain with the force of the water. One of the better products we have found is made by Zinsser. It is called Jomax stain, mold and mildew remover for decks. It is a product that , when sprayed on out of a spray bottle, will immediatley take away the stains. Was with a garden hose and the deck comes back to like new appearance!!