Can you post larger pix?
I think Blue Eyes granite is lovely and rightfully should be the focal point. I think the marble would be too busy and have a different pattern to it that does not complement the BE.
I'd try for something rather simple to make sure the focus stays on that granite. A stainless splash or a pale pearly tone tile would be quite nice. I think the right shade of glass tile would also work. I don't think you would want tiles that fight for attention.
I'd avoid blue for the bs because you have the blue island and blue spots in the counter and they will show up more if you don't add a 3rd blue to the mix. How about a very pale green tile? Usually there are green flashes in the blue parts of Blue Eyes.... Silver is really a great color to pair with it too...

Everyone has different ideas about backsplashes but I like them relatively simple.

Some kitchens can take dark splashes but I think those are the exception. Also, blue granite is sort of it's own thing and it's relatively active and colorful. I don't see the need for a border with 2 different tile sizes as I feel that detracts rather than plays it up.

I'd look for a color in the granite itself that's complementary but not competitive and see about finding a tile in that colorway.

Another consideration is whether the granite is honed or polished. If honed, you might want a more matte textured tile; if polished a glazed. If you wanted to go blue on the splash, something like 4 x 4 glass tiles or small glass subways in one of the blue shades from the granite would give you a gorgeous light-reflecting surface even if they were relatively dark.

I like the marble, but not with the border of small tiles. On my screen, I don't see the brown tiles accenting anything that you have. I think your primry accent is the lovely blue island and granite. Let that stand out rather than compete with other powerful tiles.