It can either look really dramatic or too dark, depending on the size of the room as well as your other components. I've attached a couple of images of our work using dark tiles, the smaller bathroom we have surrounded the bath with dark tiles but then balance it with an extra large mirror to bounce the light around the room. The larger bathroom we have taken the floor colour way onto the wall but used a different texture on the walls for drama. Hopefully this gives you more of an idea but you shouldn't to nervous of the room being dark, it's where you can create a contrast as well as textures. Look forward to seeing what you go with in the end.

I love these Roman/Asian style baths, My favourite but I would make the front side high and wide enough to sit on and use this to get in and out of the bath, the wall side make flush to save space. Mosaic tiles for the tub and perhaps back wall. Not necessary for sloping ends. Great safe baths for showers as well, love it Use a shower curtain

I am just wondering whether it is a good thing or not to go with solid colours like the dark blue (prussian blue), grey floors and white on other walls all in tiles. I see that a lot of designers seem to do a pattern on one wall, but I haven't found one suitable and whatever I do, I just don't want it to look out of date in 5 years time. I am not sure how about much of the prussian blue tile is appropriate in order for it to be noticeable but not too overpowering.

I like your 2nd option best. Will give your bathroom some interest without being too overpowering. Best of luck with whatever you decide.