Dark Blue was a two-episode hentai OVA series animated by PoRo and released on April 27, 2012 and March 29, 2013. The series is based on an eroge game created by LiLiM DARKNESS.
Yukito Aizawa brings his two sisters to the mansion of his childhood friend and crush Otoha Kunoji. However, one of Yukito's sisters is mysteriously murdered as everyone stays in the mansion to seek refuge from the killer and indulge in their own pleasures.
Dark Blue's plot is mostly a backdrop to the various sexual acts shown in graphic detail involving Yukito's roommates and the boy getting arousal from seeing them occur without stepping in to halt them. What makes this element to his character rather irksome is he lets arousal get too much to his head to halt his sister being raped by her teacher and stopping his crush, Otoha, from getting nailed by Reiya.
But with this being a hentai, character depth is secondary to graphic sexual scenes playing out with all the typical foreplay you can expect with oral and anal sex, doggy style, hand jobs, breast fondling and a few other sexual positions at work.
As for the murder plot that the series eludes to, the second episode's conclusion has a poorly-explained finale revealing who the killer is without any kind of buildup or foreshadowing thanks to this title's heavy focus on its sexual antics.