I have a H.O. with a flagstone patio that has a gorgeous old oak at the outer edge.  When ever the patio was layed, it is obvious that it was never sealed(if that is possible).  There are black stains that I don’t know if it came from the oak or is some kind of mold/mildew problem.   The stains are not consistent in the various pieces as some are heavily stained and some are not that bad.  I’ve tried power washing that took some of the staining away, but not all.  I tried a test patch with bleach and nothing at all budged any more.  It only goes down a 1/16th (a piece of a layer flaked off and it was clean underneath) , so it isn’t coming from underneath.  So my questions are, what might it be, what could I use for cleaning and is there a sealer to prevent this from happening again?