I've never really liked it, but at least it was shiney when occasionally clean.
There may have been some involved and an over enthusiastic cleaning incident and it's now distinctly grey.
Not quite sure what I might have cleaned it with but <looks around kitchen desperatly> it could only be some Mr Muscle, which I don't normally use, but I am sure I have used without making black things grey before <checks cat, nope, that's still black>
So what I have I cleaned it with? toothpaste and a pumice stone?
And how do I go back from 50 shades of grey to one shade of black?
Is it actual granit? Or some other stone? Or composite?? AFAIK acids ie vinegar or red wine can discolour marble and light coloured granit. Abrasives will make a high-gloss surface look dull and make a black greyish, non?
Can you ask your kitchen installer or whoever supplied you worktop?