The Darkness was developed using Starbreeze's proprietary Starbreeze Engine; however, for the sequel, Starbreeze is out, as is its proprietary tech, in favor of some different proprietary tech. New developer Digital Extremes is using a new version of its own Evolution Engine, seen in the 2008 glaive-em-up Dark Sector. It's evidently well-suited for games with "Dark" in the title. This title marks DI's return to ... its own engine. After Dark Sector, the developer has assisted on Unreal games including the two BioShocks and Homefront.
The switch to both a new developer and a new engine suggests that The Darkness 2 could be a very different game than its predecessor. Thankfully, we've already learned that the new engine is capable of rendering "Demon Arms," so it won't be completely foreign.