Were the backsplash and grout sealed? If not, they should’ve been.

Hopefully, someone will give you advice, and you’ll be able to restore the grout. If and when that happens, be sure to have both the marble splash, and the grout sealed.

The backsplash was sealed but not sure about the grout. Our contractor said sealing does not prevent 100% of stains, it is bound to happen.

The use of a high performance grout is generally better than choosing standard cementitious grout and sealing it. But, this is where you are, so try a plain baking soda and peroxide poultice. Then let it dry. If it does not dry out and look right, then it's time to use an epoxy grout colorant over the grout. That both colors it, and seals it against grease and other stains.

I was able to get some of the ”darkness” out with a baking soda and vinegar paste but there are still dark speckles in the grout that you can see from afar. I really should have went with a darker grout color.

If you really wish you had darker grout, now you know how to achieve it...

(I'm not really kidding. Use something that won't spoil, like mineral oil. Test a less obvious spot first.)

Try Oxyclean and a toothbrush.

The Oxiclean wont eat away at the grout?

Dawn & scrub

@HU-757437066 - Oxiclean was recommended as a grout cleaner on the John Bridge Tile Forum. There's also a bunch of instructional pages and videos on using Oxiclean to clean grout, just google. You could also buy one of those little steam cleaners (a link to one below) and see if that works.

How to Clean Tile Grout with Oxiclean

Bissell Surface Steam Cleaner

The backsplash is marble tile so i do not want to use anything abrasive.

I just noticed this sentence. You should never have used vinegar anywhere near marble.

Lesson learned for next time! However i only used it in the grout and kept the baking soda/vinegar paste off the tile.