Its got a pretty cool story line, but the ending for both seasons fks with your mind. Its about this guy called Hei who follows the "Syndicate" and does various missions for them. Around half way he betrays the "Syndicate" and the story changes for the better. The first few episodes can be a bit boring, but trust me its worth it. Kinda like Lucky Star, without the comedic effect, how the first few episodes are boring (talks about food....) but after that it gets funny.
For glimpse of storyline
Darker than Black (TV) - Anime News Network