I'd finish the tile installation and then choose a grout, once you've see how the light plays with it... you may change your mind.

Here's the finished install. No grout, yet.

I would use a lighter grout to show off that fantastic pattern

The dark grout looks like it would provide enough of a contrast. I love it! Too light of grout might highlight the pattern too much. I worry it would overwhelm the space visually.

The tiles have a shine to them? If you did black grout then you would have shiny black tiles and matte black grout. I'd want the tiles to look darker than the grout and not lighter.

I'd use a grout that was light enough to highlight the pattern in the tiles. Speaking from experience, dark grouts often look good on day one, but can pick up minerals over time and start to have a whitish cast to them. And really light grouts can also show some discoloration if they are paired with really dark tiles. Since it's not uniform, it can make the installation look non-uniform. This is not an issue if you pick a grout that isn't at the extreme ends of light/dark. Also, whatever you do, using epoxy grout can lessen or prevent staining issues.