Commenting because I am interested in the answer. I have the opposite problem. New home with Granite kitchen counters and I left a drain cleaner container on the counter, it somehow got wet and, well, I chemically whitened my dark counter tops.
To get rid or a dark spot you could try making a paste from Bicarbonate of Soda (peanut butter consistency), spread that on the affected area in a relatively thick layer, cover with plastic wrap and poke holes to allow the poultice to breath.
Hope it helps