Dark areas began appearing in my pool around April of this year, just below the tile & water line. Picture attached as an example. Pool contractor dispatched their stain person. He said he couldn't identify. I tried several removal methods such as Vitamin C, Chlorine tabs, JM Stain Identification & Treatment. None completely eliminated the area and most played havoc with my water balance. What shows in the picture appears at various points around the entire perimeter of the pool but not in a consistent fashion. Also notice the dark blue discoloration in the grout lines of the tile. Any input on identification and corrective actions is appreciated.