Conventions seen in The Black Dahlia

This scene conveys the information that this is a crime drama; guns and death and the binary opposition of good versus evil.
The desaturated colour of this film is a nod to the black and white era of the events of this film but also a way of updating film noir for the 21st century.

This over the shoulder shot has the woman Kay in the centre indicating a potential love triangle and cause of conflict between the two male protagonists. Lee [on the left] is looking at this friend but Bucky is looking at Lee's girlfriend, clearly showing his interest.

Another example of the position of Kay between the two men.
Note that they are all smartly dressed, jackets, ties, a hat for Kay, indicating that events have not yet taken a turn for the worse.

 Here we meet Madeleine, who becomes Bucky's temporary love interest while Kay is unavailable due to being Lee's girlfriend and whose friendship is more important to Bucky than taking advantage of her interest in him.
Note the difference is the way these two women are dressed. Madeleine dark haired, alluringly dressed, all in black in a provocative stance while Kay blonde haired always in very light coloured, high-necked clothing in a similar stance yet with very different effect on the viewer and role in the narrative.

Both are smoking but this was more a feature of the time than of their roles.
Kay is more often the subject of high-key lighting connoting her innocence while Madeleine is more often low-key lit with the chiaroscuro shadows casting her in a clearly threatening light. She is truly the femme fatale here.

This is another good example of low-key lighting and deep shadows clearly conveying the murkiness of the character's motives and character.

This is the scene where we understand what makes Bucky the guy with the secret, the murky past, which makes him admirably suited to operating in the shady underbelly of the American society of the 1940s.
He is about to lose a fight, deliberately, for money to save his father, a laudable motive, but he has never lost a fight before and certainly not for money. Here we see his guilt on his face and the vent grooves on the lockers behind, like blinds, just reinforce the murky world he is entering voluntarily.

 These two frames are examples of the way venetian blinds convey the ambiguities of characters' motives and feelings. The policemen are watching the porn movie in which the murdered starlet had a role. There is a sense that some are watching from a less than professional perspective, perhaps for enjoyment! While Bucky in the front clearly looks uncomfortable.

 While this frame on the left, has his confrontation with Madeleine and the gun pointing at her belly is a sign that he is coming to terms with her duplicity and makes the viewer wonder whether he would use the gun.

This still is an example of the use of mirrors in films to emphasise a character's duplicitous nature. The more reflections the less trustworthy they are. Here Madeleine is captured looking positively evil in this close up, with her down-turned mouth and hard eyed stare.

Finally this medium shot of Bucky in his unbuttoned long coat with dishevelled appearance, tie pulled down and general air of desperation in the pouring rain is a stark contrast with earlier in the film in happier times and indicates how far he has fallen.
He is now fully deserving of his character as the anti-hero.
He has fallen for the charms of the femme fatale, fallen in love with his best friend's girl, slipped from his high ideals as an upholder of the law and killed the people he should have been bringing to justice.