NEW YORK, NY.- It’s a numinous blue that suggests the sky, the sea, the infinite. It’s a field — poetic and abstract — that sets off the eloquence of dancing bodies. The cerulean backdrop associated with the ballets of George Balanchine and New York City Ballet has come to seem so natural a setting for plotless ballet that we barely notice it.

But at sunset on Wednesday its importance will be broadcast across New York City, Balanchine’s adopted home: The Empire State Building will light up with “Balanchine Blue” to celebrate the 75th anniversary of City Ballet’s first performance, on Oct. 11, 1948, at the City Center of Music and Drama. And at Lincoln Center, the company’s home since 1964, the company, founded by Balanchine and Lincoln Kirstein, will perform the program that it danced on that first night: “Concerto Barocco,” “Orpheus” and “Symphony in C,” the first and last set against a blue background.

The color is “a blue with little or no green in it,” slightly shading toward purple, said Perry Silvey, who worked in City Ballet’s production department from 1977 to 2019.

But it’s not just one color. “The blue in ‘Agon’ is a different blue than ‘Symphony in C’ or ‘Divertimento No. 15,’” Mark Stanley, City Ballet’s resident lighting designer, said referring to three Balanchine classics. (Forty-two Balanchine works in the current rep have lighting in the blue family, he said, as do about 30 not by Balanchine.)

“A deeper, richer blue evokes a different feeling from a lighter sky blue,” Stanley added. “We try to find the color that fits mood, emotion and energy of each ballet.”

Within the company, he said, the color is called “Rosenthal Blue,” for influential lighting designer Jean Rosenthal, who worked with Balanchine on many early ballets. “They were already using blue lighting at City Center,” Silvey said. “I assume it started with the idea of the sky.”

Maybe. There are other theories, too. “The ‘Balanchine’ blue was the color of the upholstery (velvet) on the seats in the Mariinsky Theater at the time he was a student,” Barbara Horgan, Balanchine’s longtime assistant, said in an email. (The Russian-born Balanchine trained at the Mariinsky in St. Petersburg.)

Later, Horgan offered another theory in a phone conversation: “A lot of the lighting and costumes changed when Balanchine started working regularly with Karinska in the 1950s,” she said, referring to the costume designer. “She loved blue. Her hair was blue!”

Or perhaps it was sheer pragmatism. “So much of what we did was because there was no money,” Horgan said. “A backdrop was less expensive than scenery.”

Even if the motive was financial, the aesthetic represented by the blue backdrop was in line with Balanchine’s desire to concentrate on movement and music, without the distractions of story, décor and costume. (Several “Balanchine Blue” works, such as “Concerto Barocco” and “The Four Temperaments,” originally had sets and elaborate costumes — and presumably different lighting — but Balanchine began paring away these elements as early as 1945.)

No one seems to be able to pinpoint exactly when Balanchine first introduced a blue backdrop, or if previous ballet choreographers had jettisoned scenery in favor of lighting alone. “You don’t get lighting by itself much before the middle of the 20th century,” said Jane Pritchard, the curator of dance at the Victoria & Albert Museum.

Stanley explained that in the 1960s, light began to be projected through a blue scrim, a transparent overlay with a blue color — the Rosenthal Blue. “Then backlighting was introduced in the late 1970s by Ronald Bates, who started to mix lighter or richer blues into the projections to get more variety,” he said.

Because lighting equipment has evolved so dramatically over time, Stanley said, much has changed. “I try to be true to the formulas I inherited,” he said. “But it’s not exact. It’s in the family.”

He pointed out that City Ballet uses a gray, rather than a black, stage floor. “That’s iconic Balanchine too,” he said, “and it changes the way you see the Balanchine Blue. That lighter floor and cool background forms what people think of as a Balanchine aesthetic.”

Sara Mearns, a principal dancer, said that as soon as she joined City Ballet she noticed that “you never felt you were being bombarded by the lights — you could just think about the dancing.” For her, “Serenade” epitomizes the Balanchine Blue lighting. “I don’t think I have ever felt more comfortable onstage than in that lighting,” she said. “You feel like you are being hugged by the environment.”

Balanchine’s use of a blue backdrop became an unspoken standard for neoclassical ballets in the 1960s and ’70s, said choreographer William Forsythe, who paid homage to it in “The Vertiginous Thrill of Exactitude” (1996), projecting the words “Sky Blue” on an Yves Klein-blue backdrop — “an expression of my admiration,” he said.

The blue backdrop is less fashionable now among contemporary choreographers, but it remains an essential part of City Ballet’s identity. As Pritchard put it: “When the curtain rises on that blue, you think, Yes, I am at New York City Ballet!”

This article originally appeared in The New York Times.