In-universe explanation: the black details could be representative of different metals they used for doors, flaps and other pieces.
Real-world explanation: a figure that's just all grey/metallic is just a little bit boring. And it would clearly be a "special edition"-figure. Like this, the figure could fit with other characters. Unlike figures like the gold stormtrooper, which would stand out in a display as marketing gimmick.
Is there an explanation - in-universe or real-world - for why it's pretty heavily weathered?