Depending on the light conditions the chain rings will look darker than the cranks and the STI levers also look dark at times.

The heavily curved surface dosn't reflet the flash back to the camera particularly well, leading to the dark look, rather than the material or finish.

Black Chrome has nothing to do with Anodizing.
Anodising is a process to modify the oxide layer on the Al, Chroming (Black or otherwise) is a coating with a dissimaler material (chrome- generally over copper). Black Chrome is a finish like the new XTR, but is UV unstable.

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For many years Shimano have offered parts in various colour finishes for the home market,aka Japan, and at times if a product is very new, in this case the D/A crank, they will offer to the teams what is available in stock.
Its possible that this photo is of a pewter coloured crank arm, which is in fact a shade slightly darker than the standard issue one.
I remember that there was a time when you could buy 600EX parts in at least three different colours, polished aluminum, polished bronze, or polished blue, so it could be a colour variation we are seeing here.

the 7800 crank has definately been product in different "colours" i have the dark grey as seen in the picture and my buddy's is lighter. i scratched the back of mine and it is a coating.