It's a great look. I have also found that when the same material is used for a top and backsplash, the vertical backsplash shows darker. The reflectivity of the polished stone will help to keep the overall lightness found in the horizontal top.

I would go with a light gray or white glass subway tile myself ... but they will look nice together.

Yes you could or another alternative is to use honed on the splash and a tile enhancer to bring out more sheen in the stone. Are you aware of the cons with using a marble on a kitchen countertop? Calcutta is a beautiful stone, however unless you are very diligent you may very well have staining down the road. You may want to consider alternatives to marble such as quartz or dolomite.