I'm not sure I even realised noir was part of Eberron, but I guess I haven't looked too much into it so may have missed it when reading through what books I have.
In the 3.5 core setting book it is mentioned a few times but not hugely emphasized directly to start, more implicitly. It gets more explicitly fleshed out in later editions.
From the 3.5 Core Setting book
Page 249, the very beginning of the chapter on An Eberron Campaign:
"The world of Eberron has a slightly different flavor from traditional high fantasy. With a blend of swashbuckling, over-the-top action and dark urban mystery (fantasy noir, if you will),"
Page 251 under Styles of Play:
"Things Are Rarely as They Seem
Intrigue, disguise, and deception are staples of the noir world. You can apply this concept to your adventures in a multitude of ways to throw an obstacle in the heroes’ way, or feed them false information that sends them in the wrong direction, or have them discover when all is said and done that they had been duped into working for their enemies."
Page 255:
"Intrigue scenarios can often be morally ambiguous; in this respect, they draw heavily on the noir tradition. If the heroes don’t get involved at the request of a trusted patron, they may have difficulty telling the good from the bad."
The 4e Player's Guide:
Page 6 Life in Eberron:
"Welcome to the world of Eberron! This book is your guide to exploring everything Eberron has to offer. The EBERRON® campaign setting abounds with sinister conspiracies, a mysterious Prophecy, and constant opportunity for exploration and adventure. The setting might be described as fantasy noir, combining pulp action with dark fantasy for a unique experience at the game table."
Page 24:
"Investigation
Eberron is sometimes described as fantasy noir, and an investigative adventure is one of the best ways to experience this aspect of the setting."
Page 42:
"Eberron is a world of magic and wonder, of intrigue and suspense, of pulp adventure and noir mystery."
4e Campaign Guide
Page 10 under Campaign Themes and Urban Intrigue:
"If you want a campaign focusing on mystery, intrigue, and politics, inspired by classic film noir with a fantasy twist, Eberron’s cities are the place to do it."
Page 19:
"Eberron’s urban centers offer a wholly different kind of adventure experience, springing from the setting’s roots in the noir tradition of film and literature (see “Fantasy Noir,” below)."
Pages 19-20 have the biggest description of Eberron's noir aspects:
Fantasy Noir
The EBERRON campaign setting is sometimes described as “fantasy noir,” and an urban intrigue
campaign highlights those noir elements. But what exactly does that mean?
The answer lies not in the details but in the broader view. The tone and attitude of Eberron make it unique—a combination of traditional medieval fantasy, pulp action, and dark adventure. The world unquestionably contains utter evil; pure good exists, but it can be tough to find. Primarily, an EBERRON campaign is about shades of gray—everything and everyone has more than one motivation and tends to obscure true goals with both attitude and action.
Things Are Seldom What They Seem: You can’t judge individuals by their appearance. Heroes and villains come in all shapes and sizes, all classes and races. A villain might help the heroes if it suits her purpose, and an ally might betray his friends if given the right motivation. Everyone you meet has a personal agenda, and until you understand what that is, you can’t say with any certainty where that person fits in the greater scheme of things.
Surprises, deceptions, and misdirection are the norm. This atmosphere makes it even more unexpected when the heroes discover creatures that have pure motives or ancient sites dedicated to a single, good purpose.
Sympathetic Villains: All heroes are measured by the opponents they face and eventually defeat. In an EBERRON campaign, though, those villains often reveal as much about the heroic characters as about themselves. A good noir villain is sympathetic because he or she is very much like the heroes, and shows what they might well become if something were to push them to an extreme.
The classic example of this kind of villain is the crusading paladin who so zealously pursues evil lurking behind every corner that he grows blind to the evil in himself—the evil that drives him to persecute innocents. The Church of the Silver Flame (page 245) seems prone to that sort of extremism, but the church is not alone in harboring such fanatics. A better mirror for many player characters would be another group of adventurers—greedy, money-hungry treasure-seekers who don’t care whom they work for, as long as they get paid. The player characters might actually cooperate with a group like that on one adventure, when their employers happen to share a common goal. When the characters find themselves at odds with their former allies, however, they quickly discover that these opponents have no qualms about resorting to the dirtiest tricks in order to get ahead. Will the characters abandon any principles they might hold to keep up with their rivals? Or will they lose the battle but win the moral war, keeping their principles intact?
Unhappy Endings: In a noir setting, victory is rarely complete. This does not mean that the heroes cannot overcome their foes or reach their goals. However, it does mean that it is perfectly fitting to work unexpected twists and details into the resolution of an adventure.
A villain, once defeated, might be revealed to have a truly noble motivation, such as protecting a family member or stealing money to pay for a child’s treatment by a House Jorasco healer. Or the miscreant’s defeat could pave the way for a new, more terrible villain to come to power. The characters’ actions might also have unforeseen consequences for their NPC allies, ranging from a change in social position to threats to life and limb.
Gaining a desired item or boon might come at an unexpected price. Perhaps the heroes must give up another prized possession to secure the one they desire. Or it might be that while the heroes were out achieving this goal, a rival was stealing something even more precious from them back at home. Every success has its cost, and every goal reached reveals itself as merely a step on a longer path.
5e Eberron Rising From the Last War
Page 5:
"Eberron embraces swashbuckling action and pulp adventure while adding a layer of noir intrigue. Stories don't always end well, and there isn't a perfect answer to every problem. The Last War turned old allies into bitter enemies and destroyed an entire nation, leaving behind terrible scars. Crime and corruption lurk in the great cities of Khorvaire. Hidden dragons shape the course of history, and sinister fiends influence the dreams of the unwary. Yet mortal greed and ambition might prove more dangerous than any dragon or fiend. This darkness affords opportunities for a group of bold adventurers to make a difference-for better or for worse."
Page 8:
"NOIR INTRIGUE
Eberron draws inspiration from noir and hard-boiled fiction. It's a world where stories don't always end well, and where there isn't a perfect solution for every problem. In developing characters or stories in Eberron, consider the following concepts."
It then has sections on Regrets, Debts, Personal Motives, Shades of Gray, and Something to Lose.
Page 183:
"As discussed in the introduction to this book, pulp adventure and noir intrigue are two major themes that interlace in Eberron. Weaving these themes together, or exploring either one in isolation, can give Eberron stories a unique feel. Adventures that take advantage of those themes help to reinforce a sense of place, immersing the players (and you) in a world that is still reeling from the effects of a hundred years of war, that is shaped by modern ideas and aesthetics, and that is set apart from the rest of the D&D multiverse by a magical barrier."
Page 184:
"SHADES OF EVIL
Moral ambiguity is a feature of the noir genre that can add richness and complexity to an Eberron adventure. In a world where a red dragon isn't necessarily evil (and a bronze one isn't necessarily good), it's impossible to know who you can trust, and even the actions of a so-called villain might not fit neatly into clear-cut definitions of good and evil. Even adventure villains are more often driven by human motives than by cosmic concepts of good and evil. People sometimes do evil things for good reasons.
Exercise some caution when exploring these noir themes in the stories you develop. It can be fun for some people to wrestle with moral quandaries in adventures, but it doesn't work for all play groups. In particular, if there's no good solution to the quandary-if the characters have no choice but to follow a course of action they abhor-some players might end up unhappy."
Page 187:
"INTRIGUE
Stories in the noir tradition tend to start slowly and build gradually as the mystery gets deeper and more convoluted, until the situation erupts in a climactic confrontation. In contrast to the pulse-pounding action of a pulp adventure, noir typically relies on intrigue-in every sense of the word-to keep the protagonists engaged as they move deeper into the story."
Page 239:
"Sharn can be a grim place that makes good people bad and bad people worse. In its mean streets, gritty detectives contend with corrupt law officers while trying to foil the efforts of merciless gangs and monstrous thugs. In the highest towers, wealthy barons from the dragonmarked houses make shady deals with crime bosses, each of them making profits by exploiting both the ordinary folk and the well-heeled upper crust alike. Corruption can be found among the ranks of government, the watch, the temples, and large businesses. This atmosphere makes Sharn a perfect location for adventures inspired by the noir genre."
Page 249:
"The true power of the Boromar Clan lies in its institutions: watch officers who have been allied to the clan for generations, judges who are blackmailed into compliance, and officials who receive lucrative bribes in exchange for their cooperation. Adventurers who oppose the Boromars might discover that people they think of as friends and allies are in the pocket of the clan. The Boromars are villains in the noir tradition, and adventurers determined to tangle with them will be drawn down into the mud."
