I'll tell you I have heard that black granite can be tough. You can use the Stonecare PRO Signature ENHANCE & SEAL Color Enhancing Sealer (16 oz.)
to darken the color. I would suggest having the customer get a plumber to remove the fixtures first so you can have full access to all the areas of the granite in case you need to try different approaches. (We never remove or replace the fixtures just because of liability issues. That is up to you.)
Hope this helps.
I just looked at a small granite slab in a bathroom vanity. The granite is discolored around the hot and cold water fixtures. It looks like water marks. It feels etched. The granite is lighter in color in those areas. How can we remove these marks and prevent further discoloration?