"Science never sacrifices itself. It is always murderous."
- – Jean Baudrillard
Ever wonder how Chaos Space Marines get ammo for their Bolters? How they repair their armor when it gets inevitably damaged? Who maintains the Traitor Titan legions? Look no further, because it's all done by the Dark Mechanicus.
As Horus plotted his rebellion, he knew that having the Adeptus Mechanicus would be a major advantage against his father. In an effort to buy them into following him, Horus promised them the Standard Template Constructs in possession of the Auretian Technocracy, and access to the Vaults of Moravec. Infected with Chaos-tainted scrapcode upon opening the Vaults, Fabricator-General Kelbor-Hal declared for Horus, and subsequently slaughtered everyone on Mars who didn't. When the Traitor Legions arrived in the Sol System, Mars was used as their staging point for Terra. Following the end of the Heresy, the Dark Mechanicus continued to function, supplying weapons and technology to the Traitor Legions. In return, powerful Chaos Lords such as Abaddon keep them in line with their shiny hoards of Archaeotech they found during the Great Crusade.
In opposition to the Adeptus Mechanicus, the Dark Mechanicus favors exploring technology to its limits and trying to create new technology. This often means finding better ways to create Daemon Engines, and making even weirder Warp-based tech and weapons. They even managed to make Servitors out of that which must not be named called Death-Masques, back when they had been derided as largely useless.
The Dark Mechanicus considers the Omnissiah to be a combined aspect of Chaos Undivided, holding it as a means of progress and innovation. However, it also contains all the drawbacks one would expect of Chaos. Instead of machine-spirits, the Dark Mechanicus uses bound Daemons to provide the intelligence and power sources needed to run the machines. In addition, the Dark Mechanicus has techno-organic technology. So everything they make is not only very much alive, but it also wants to kill you. More than usual, we mean.
But then again, if you're a Chaos Space Marine, you're fucking badass and it shouldn't bother you. Member of the lost and the damned, eh....
Unlike the AdMech, the DarkMech hasn't been running around the galaxy all that much. While on paper, they should have evil Skitarii, Titan Legions, and warships at their command, they haven't been out with the rest of the forces of Chaos to rape, pillage, and burn the Imperium. However, instead of sitting on the couch for the past 10,000 years, they have been operating the daemon-forge worlds within the Eye of Terror and beyond to keep everyone armed and happy.
A good look into the Dark Mechanicus in the fluff is to read Ben Counter's Dark Adeptus in his Grey Knights series. Seriously, check it out. It will give you a good idea what kind of demented madmen fill the ranks of the dark order. Also Graham McNeill's Dead Sky, Black Sun from his Ulltramarines series; in addition to series' overall depiction of the Ultramarines not being complete faggots, a few chapters show what life might be like underneath Dark Mechanicus. Aaron Dembski-Bowden's Night Lords has a Dark Mechanicus who evidently decided to do as the Romans do and try to look as intimidating as hell by coating his skeleton with steel and wearing a black robe over it, so he looks like a steel-boned Grim Reaper. He also invented a jamming device that runs off of torturing a Titan pilot and broadcasting the resulting binary screams at max power. The Adeptus Mechanicus Duology is also set on a DarkMech forge world, and gives you a nifty insight into how they function and live.
Well it wouldn't be far off: since the Adeptus Mechanicus got two Codexes (one for the Skitarii and one for the Cult Mechanicus), it wouldn't be surprising. It also would be pretty cool because of all the different stuff that would be added in - either absolute cheese or some pretty nifty Daemon engine rules, say finally getting Stalker tanks. Or better yet Servitor daemon hybrids.
Forge World has heard your prayers as the Dark Mechanicum will finally have rules in Horus Heresy: Angelus. Ave Omnissiah!
With the release of Blackstone Fortress, there are finally plastic dark mechanicus models, sort of, in the form of the Negavolt Cultists.