According to dojin mangaka Guyohmaru's blog, Aniplex announced at a preview for the anime business people that four extra episodes of Darker than BLACK - Ryuusei no Gemini
will be released as DVD/Blu-ray only episodes. The four episodes
are the side stories bridging the gap between the first and the second series and will be included in the even number volumes of the DVD/Blu-ray Discs. The number of the main episodes to be aired on TV is twelve.
Source: Guyohmaru's Kakunoko
Updated on Oct 8th
According to Neowing, the first extra episode will be released on January 27th.