I guess I'll copy the stuff I crappily translated out from Newtype + the DtB website over here.
I never realised there was a prelude in the official site though, that's where the animenfo synopsis was translated from. It seems that these "keiyakusha"="contractors/contractees" (I don't know which of the two it is) have got supernatural powers (this is mentioned in the scans as well) which is related to the Hell's Gate.
But what I want to know is, what's with all the Chinese? Surely they're not going to put something stupid like saying that the Hell's Gate is really a conspiracy of the Chinese government or something, right?
Anyway, the following info is taken from Newtype Jan. and also from the Darker than Black website both.
It's not all direct translations, I kind of threw them both together and picked out what I understood. I don't think I left out any details though.
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Characters
Spoiler for saving space:
The main character, 黒 (Hei (Black)) is some super top-class agent, at The Organisation (well, it's just called plain old 組織 in Japanese)
He's one of the best in this group of people with supernatural powers, called "keiyakusha". As such he's an invaluable member of The Organisation...
The scan describes him as a 他人に心を許さない孤独な男 - a solitary guy who won't allow others in his heart.
Currently he is posing as a timid Chinese university student called 李舜生 (Li Shun Sheng, if I am not wrong).
The seemingly lead female, 銀 (Yin (Silver)) is a support staff in Hei's team. She seems normal but she's got absolutely no emotions at all and is sometimes called "Doll". She's also blind, but her other senses have been honed to such a level that she can... instantaneously understand the physical situation around her for a radius of 50m (半径50m以内の状況を瞬時に把握する)
Then there's your customary oyaji, 黄 (Huang (Yellow)). He's the team's manager and supervisor, collates information and acts as the team's connection to The Organisation, but he is not a "keiyakusha". (Going by the site it seems that he's a bit jealous of them, too) His age is unclear but it seems that he's younger than he looks. He's the type of guy who'll do anything for money.
And finally there's 猫 (just plain ol' Mao - means cat.). It can talk and acts as the team's information gatherer... but that's because he used to be a man - and a "keiyakusha" to boot - but for the time being he's posessing a black cat and I think he's lost his original body (本来の肉体を失ってしまった) so he can't revert back. The parts of his mind that couldn't be supported by the cat's brain are stored on a LAN server where they're connected wirelessly to the brain...
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Story
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Production
Spoiler for Saving space:
The character designer, Yuji Iwahara 岩原裕二, had this to say: "It might possess a very complicated sort of worldview, but this is ultimately about human drama. It's about investigations being carried out by underlings; and we are going to make the interactions between Hei, who's at the bottom of the hierarchy at The Organisation, and each episodes' guest characters a regular feature (組織の末端で働く黒と、毎回のゲストキャラの交流をメインに描いていきます)
Iwahara, who is by profession a mangaka and is doing his first work in anime production, will of course have to draw a lot of guest characters.
"I was asked [to join this project] because I'm the sort of person who draws lines a little differently from animators. It might seem a bit like manga, but they're also characters designed to be moved realistically and have appeal at the same time." (「アニメーターとは違う線が描ける人ということで、お願いしました。漫画っぽいけれども、理屈にあった動 きができる、すごく魅力的な人物を描いてくれるんですよ」)
There were many difficulties because everything from the characters to the atmosphere/worldview had to be created from scratch [or so they say, anyway], and furthermore while the number of anime has been continually increasing, less and less of them are original projects [i.e. not based off manga or an original work (gensaku)]
"While I do think it's true that it's easier to make things based off existing material, I also believe that for all the extra work that you have to put in, you should get a correspondingly more interesting and unique result. That's something I would like to express, I think" [I don't know who said this, it's probably the director or character designer.]