The Noir detective genre was created in California of 1930-1940s. It just means ‘Dark’ so it was related to the black and white movies of Sam Spade and Philip Marlowe.
In modern days the Noir terms have been used in contemporary world fiction like; Icelandic noir, Nordic noir and recently Brighton Noir.
Noir is just dark. If you think about the colours grey, black and the pollution on city streets with a sick and lower class of people on the streets.
Usually, the protagonist has a flaw that impinges them in different ways. This makes it fun to explore the human condition and hidden fears or torments that we find in us all.
It will be a necessity to become comfortable in adopting a gritty and direct writing style.