Darker than Black follows the story of Hei, a contractor who is tasked with various missions for a mysterious organization known as the Syndicate. The series progresses as we, the viewers, watch Hei executing jobs for the Syndicate, and bit by bit, we piece together the rules and mysteries of the world of Darker than Black.
DtB is like an edgelord: it's full of angst and melodrama, but it's also surprisingly intelligent and thought-provoking. The series does a great job of creating a complex and believable world, and its characters are well-developed and relatable.
However, not unlike edgelords, the series can also be a bit pretentious at times, and it takes itself a bit too seriously. The plot can be convoluted at times, and the series can be a bit slow-paced at times.
Overall, I still consider it one of the best series of the 2000s, and the creator of the manga deserves a special mention because the man has a hand in every legendary anime you can think of, which makes him a legend, no doubt about it.
You should watch it; you won't feel an ounce of regret.