Some boards appear to have discolored streaks. They are commonly referred to as mineral streaks.  This happens when dark colored minerals are drawn into the tree from its water source or when mold grows in poorly ventilated areas.  The stains are generally blue or brown.  Stains can be cut out or covered by a dark colored wood stain.  Some commercial wood cleaners containing oxalic acid can be effective to clean off stains from mold.