In addition to Richard's suggestion, I'm wondering if the darker areas might also have a bit of a fine texture and the lighter areas a bit more polished. The overall hue of the blue is in the same family for both the light and dark tones. Can't tell from the photo, but a fine texture won't return as much light, hence a darker tone.
not having seen any of his work up close and personal, i'd guess at least some areas are selectively polished and/or blasted. surface finish can have a radical effect on apparent color. and as i've learned the hard way over the years, sometimes i've tried to recreate a tone that only ever existed in the photograph, not on the actual article.
perhaps all of the above. maybe the solutions for the different colors had a slightly different % strength. such applied to the varied surface finishes previously mentioned might do this. different patination solutions will produce a variety of shades, all depending on solution strength, temperature, and application time. the patination is no guess, i've done it many times. the bluing is something i have done is "straight out of the bottle". good luck in your quest for this info. do you wish to do this , or are you simply curious?