Satin-black spray paint, perhaps?
Astonishingly, that's what made my Zéfal HPXs black, when comparing them to the silver versions. Came from the factory that way. A bit of Scotchbrite pad to give the silver shaft some "tooth", paint, and you're there.
I'd thought them anodized* but nope; mine were all painted, and by Zéfal.
Best,
Dan.
EDIT: The older silver ones were anodized. I think the more recent ones are polished, but haven't seen one up close.