Usually this is the result of converting copper sulfate stains (turquoise) to cupric oxide (grey-black) by oxidation from the shock (calcium hypochlorite) sitting directly on the copper stains. Avoid this by shocking with liquid chlorine or pre-dissolving the granular shock so it doesn't sit right on the plaster. Often the grey-black stains can be reconstituted to turquoise sulfate by letting alum (aluminum sulfate) sit on the same spot. This has to be done relatively soon, though, before calcification covers the stains with a layer of carbonate that the alum can't soak through...
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