Given the extent of the apparently solid leaks of the gold or champagne color, it would be odd for there to have been no prior leaks of a fourth color. There are several alternative explanations for what is apparently shown here.
First, this type of photo is very easy to fake, either in Photoshop or by overlaying readily-available black glass on a white iPhone. Second, it could be a genuine Apple prototype from a stage at which Apple was trying out other colors. Third, it could be a quality-control failure. Finally, and most likely of all, it could be a counterfeit. Counterfeit iPhones have been around in China for years (hell, there are even counterfeit Apple stores), some of which attempt to be close replicas and some of which use different colors. It’s a near-certainty that there are already knock-offs in existence based on the 5S.
Even lighting and angle can create this kind of illusion. The photo below is simply a standard white iPhone 5 which appears very similar in appearance to the claimed graphite 5S without any alteration:
This doesn’t appear likely to be the explanation in this case, as we see the phone from several angles, but it does serve as a useful reminder not to get too excited by such claims.
None of this means the photos couldn’t be genuine, but that’s certainly not where we’d be putting our money. Either way, we don’t have too long to wait: the iPhone 5S is expected to be unveiled at a media event on 10th September and go on sale on the 20th.