Personally, the GMT Master II with black and blue Cerachrom bezel is one of my favorite current production Rolex watches. And while I could never see paying extra money to coat it with a DLC treatment that effectively devalues the timepiece, and voids the factory warranty. The blacked out GMT Master II looks pretty badass, especially with the black and blue ceramic bezel.
Similar to Bamford, Ember Watches takes a factory Rolex and turns the 904L stainless steel surface to black with their own special coating. According to Ember, their process involves staining, polishing, sand-blasting and pearl-blasting the surface to achieve the black color. They can also customize the dial.