On one side Sunset Boulevard is a homage to Hollywood with its love letters and reverences to some masters of the industry from Buster Keaton to Erich Von Stroheim. Wilder is really self-aware of this by for example, making Stroheim (the director of Gloria Swanson's early films) play Max, who in the film is also the director of Norma Desmond's (Gloria Swanson) early films or also a nice hat tip to the own career of Swanson by making herself depict her own come back or also she homaging Chaplin another director she worked with makes for some hearty obeisances.
However, on the other side, it is also a hefty criticism of film industry as well. Wilder's approach is neither shy nor discreet on his aim to portrait the price of fame as self-deceit, spiritual & spatial emptiness, greed, selfishness, manipulation, and ambition "paint the picture" by themselves. Right off the bat we know how this story is gonna end so the mood of the film is permanently on decay.
While color has been presented on film pretty much since its earliest stages (thanks to Technicolor as for example in Fantasia or The Wizard of Oz) during the epoch that Sunset Boulevard was produced color was already a common choice to go with. Wilder took the choice to shot on black and white on purpose but I would disagree with the statement that he did so to "recreate a noir". While it is truth that noir films are generally more characteristic for their black and white cinematography, that is not its exclusive characteristics, so indeed a noir can also be shot in color (Niagara, Vertigo, Foreign Intrigue, etc.) Despite any choice of color usage Sunset Boulevard uses a lot of common noir tropes like the femme fatale so I wouldn't point to recreating a noir ambiance as the main reason to pick up black and white.
I would rather point at what John Ford stated:
"In black and white, you've got to be very careful. You've got to know your job, lay your shadows in properly, get your perspective right, but in color, there it is. You might say I'm old fashioned, but black and white is real photography."
Although shadowing and lightning is an important aspect of the visual composition of any film, it is on black and white where it is absolutely crucial to the point that a mere change can make the mood and meaning of the scene completely different from what is intended. Throughout the movie this is prominently used, cinematographer John F. Seitz (who already worked with Wilder in Double Indemnity) handles to bring together the light and the dark without any joint showing giving further importance to the visual contrast of the "luminous" outside world of Norma's "palace" and the gothic and gloomy mansion.
Joe outside the mansion
Norma and Joe inside the mansion
And the final shot and scene, as Norma goes down stairs, symbolically "descending" from the stardom to then be blurred and faded away from the camera, only black and white could have gained to make all those moments so visually meaningful and effective.
