I personally would be extremely claustrophobic in a shower like that with dark tile. It will be a cave. Dark tile will also show everything so thats a lot of cleaning on top of it. A medium gray tile would probably will achieve the look you're going for without being too much maintenance or too dark.

Also remember that in your inspiration pictures they used professional lighting and editing. You really need to think about lighting if you're going the dark route.

That’s a very good point and the main reason I have been having doubts. In the shower itself, it will have two chromotherapy lights but not enough to avoid the cave-like feeling. A lighter grey and light/white flooring would be your suggestion then?
Thank you for taking the time to respond!

What does this have to do with a pool?

Where did the pool come from. I actually like floors and shower walls the same , too many finishes in bathrooms are the reason people want a change. I sure hope whoever is doing the steam shower has very good credentials this is not a job for the guy who does basic tiling and install . Maybe post some pics of what you have chosen or at least are thinking about .BTW I much prefer porcelain tile for showers steam or regular.