Black is much too high contrast and busy (unless that is what you like). Either a white, off-white, or a very pale gray close to the white of the tile would be lovely and subtle.

White or soft white to blend with the tiles. It's classic and always on trend (just like your kitchen look and white subway tile). Picking the right grey will be tough and black is too severe and busy.

I LOVE the contrast of a dark grout with a white tile. I think black would be too dark, but a charcoal or light grey would be awesome! Case in point...see 1st photo. This is where I fell in love with the idea of a darker grout. White on white is done over and over again...break the mold!!! I included a second picture of a dark grout used in a kitchen application too.... You already have so much white in there, I think a darker grout would really add some dimension and interest!

I know this is an old post, but for the sake of anyone wanting more info on this subject...
I'm preparing to have my kitchen in my 1912 house remodeled. My designer liked my idea to have white subway tile with dark grout to make it look more period, especially since my hardware is going to be a dark (oil rubbed) bronze. He suggested a charcoal gray grout rather than black because black shows imperfections in ceramic tiles and workmanship more than the gray. Personally, I look forward to not having to clean white grout when it starts getting dinghy.