Did the plaster turn black because something landed on it (settled) or is it discoloring the plaster itself? Try brushing and see if it is “in” the plaster or just on it. If on it, vigorous brushing, lowering the pH and alkalinity will allow the filter to trap and remove sediment. If it is metals that were forced out of solution you may need to add a metal sequestering agent, with a sand filter, our FerriTabs will take out trace metals including iron, manganese and copper. SeaKlear natural clarifier also helps any type of filter work. If the stain has deposited on the plaster and will not brush away, ascorbic acid like that found in StainFree will help lift the stain from the plaster, and then the sequestering agents mentioned above will help remove it.