go with gray.

match the tile color.

can you post a pic of your blue countertop? Is it blue macabus?

What about blue to match the counter?

white grout gets dirty so fast and you'll never manage to keep it clean. gray grout looks classic and makes a vintage-looking tile like subways (especially set close together) look like they've been there forever. the darker grout colors also really accentuate the running bond pattern of the subways.

doing a primary color like blue just seems really risky--like it might look really dated and 80's-ish in a few years.

What about blue to match the counter?

I was going to say the same thing, but rather than trying to match it use a more subtle color, like Custom's Arctic Ice. With the white tile, it'll show up almost as a light sky blue-- just enough to bring out the pattern in the tile without being overwelming, and blue enough to tie it into the countertop.

I would go with a light grey. The thought of blue grout makes me cringe! JMHO Below is an old thread discussing spacing and grout color.

Here is a link that might be useful: Old thread on spacing and grout color

I don't want blue grout -- there are places for excitement and color, but tile grout ain't one of 'em. After reviewing the old thread (thank you! that was very helpful!), I conclude that a slightly darker grout helps separate the tiles, while same color grout creates a monotile look. I prefer the separate tiles look -- JMHO. Last question: Why grey instead of off-white/beige/ivory?