We have blue black slate and my partner also cleaned it with something so it took on a white-ish dullness, tried everything, hands and knees scrubbing with all sorts in the end we decided to varnish the slate! It now looks more black then the blue tones it had but they are bright, shiny and clean looking. They looked "wet" for a while but this has died down a bit. Not sure if it recommended to varnish slate but we borrowed an industrial buffer first to bring them up best as poss before varnishing. Think he may have used a special varnish... good luck
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