I think black slate, satin finish, is sufficiently neutral that it will blend well with any decor, whether traditional or modern or in between.

I just responded to your later post, but if you are still deciding on materials, you should see the site below. Gorgeous fireplaces, almost all with black slate. You could use slate tiles for the same effect. See the Soane collection under "Products"

Here is a link that might be useful: Chesney's

If you want a lighter color, travertine or granite tiles or custom cut slabs work well.