One morning recently Stella Blum was on the subway, riding from her home in Brooklyn to her job as curator at the Costume Institute of the Metropolitan Museum of Art. A man sitting across from her was wearing two pairs of jeans (the pair on top cut off and slashed to reveal the pair underneath), an Eisenhower jacket, a couple of scarves and a hat.

To most passengers on the train he was one more subway bum. But not to Blum. "He added up esthetically in a funny way," she said later that morning. "There was a quality and an integrity about his appearance that beats what some women pay thousands of dollars for and end up looking like they are wearing two hats."

Blum's ability to look at clothes and derive a sense of the wearer and the period in which the clothes were worn is one of the rare skills she has brought to the Costume Institute. Besides making the institute an important yet comfortable place for study and research, Blum has assembled 11 remarkable costume shows, including "Hollywood," "Russian Style" and, currently, "La Belle Epoque." At the end of June she will leave to become a professor at Kent State University in Ohio and curator of its newly acquired costume collection.

In her 30 years with the Metropolitan Museum, Blum has helped bring about a change in the way people look at clothes. "It has become a valid discipline in the field of decorative arts," she says. In the beginning, she recalls, people walked in and looked at clothes and said, "Isn't that amusing? Could you imagine wearing it?" Now they say, "How times have changed." Adds Blum, "Now they want to know what situation created the costume, how the role of women has changed. They want to know more. They came to the Russian show, for example, because they wanted to learn more about Russia."

"Clothes are the most personal and perhaps the most valid record of man's view of his society," says Blum straightforwardly. She rarely talks in curatorial lingo and never assumes scholarly garb. The other day, she was wearing dark gray flannel trousers and a paler gray sweater, which she hand-knit. Her boyishly cropped hair is paler gray still.

"Costume, like almost everything that is creative in life, started from something practical," she says, using as an example the Greek amphorae once used for wine and oil. "Humans are endowed with a creativity of knowledge; maybe because of the knowledge of death, we try to enhance our lives while we are here.

"Even in climates where you didn't need clothes, they became a way to protect yourself, maybe against the evil spirits, or a way of filling a social or physical or psychological need. Clothes respond to a need with something practical, then take on a decorative quality."

Blum first started looking at clothes seriously in 1940 when the U.S. Employment Office in Manhattan sent her to see about a job as receptionist with the Museum of Costume Art, then in a suite of rooms at Rockefeller Center. At Syracuse University, where she graduated in 1938, she had been an art major. Her only fashion-related course was a senior-year elective in fashion illustration. In 1942 she took a sabbatical from her first job at the Museum of Costume Art to look after her small children and to continue her studies, developing her own broad fashion studies program at a time when none was available. By chance she submitted a dress design to a contest in Chicago. There were two winners: Blum and Bill Blass. He went on to design, and when Blum's children got a little older she went to work at the Metropolitan, which by then had taken the Museum of Costume Art under its roof. In 1960 the Costume Institute became a full department at the Metropolitan. Blum was named its curator in 1972.

The lavish annual costume shows she has mounted have proved to be a major attraction at the museum. She has trouble pinpointing a favorite. "They are like your children," she says, but then decides that the show focusing on the inventive clothes of the "1910s, '20s and '30s" might be it. "It was the first time we examined designers as artists. If you looked deeper, if you pulled the skin off the visual effect that the show had, you could find something that spoke to you from 1909 to 1939." She says the Poiret clothes of pre-World War I exemplified the changing role of women, and the humorous designs of Schiaparelli gave a clue to the nervousness that preceded World War II. "With what she did with bustles and big sleeves you could tell that times were unsteady," says Blum.

In the Russian exhibition, Blum loved the mix of clothes of the royalty, the nobility, the rich and the peasants. Recalling its influence on Yves Saint Laurent in 1976 and on others today, she says, "Endless designers came here with sketch pads, but nobody wanted to copy Catherine the Great's wedding dress--rather the rich peasant clothes with full dirndl skirts, kerchiefs and a mix of patterns. Isn't it interesting that what was most valid for our time was the peasant clothes?"

But it wasn't only designers who were captivated by the Russian show. It was the most popular of all the costume shows, attracting more than 800,000 visitors in the eight months of the exhibition.

Blum credits Costume Institute special consultant Diana Vreeland's personality and flair for bringing worldwide attention to the exhibits, and for revving up essential support from the New York fashion industry. Blum ensures that Vreeland's "flair" never truly violates historical integrity. Vreeland's exaggerations, such as overgenerous use of jewelry or covering the mannequin's faces with stockings, are appropriate, Blum says, "because life is theater. People move. And in order to project, like theater, you must take that one extra step. Otherwise it becomes rather ordinary, just as life can become ordinary. Such showmanship assists people who are just passing through to come away with an idea."

Walking through one of the costume galleries late one afternoon after the museum had closed, her heels clicking on the stone floor, Blum glances at the mannequins in the "Belle Epoque" exhibit with a nod one might give to passing friends. But her eyes turn quickly from Queen Victoria posed seated, overlooking one of the rooms. "She was a strong woman, a Mack truck," she says with a laugh. The Queen Victoria mannequin "is very complete," she says. "Even eerie. I can barely look at it."

Finding suitable mannequins with the proper proportions and poses was as much a problem for "La Belle Epoque" as for other exhibitions. Even the shape of a face changes from one period to another, Blum explains. "The Victorian head is a little sweet face that is oval-shaped. The turn-of-the-century woman had a much more mature face with a jaw line that was well-defined because she wore collars that were boned. So you needed an older cast to the face, a squarer jaw, to project the period."

"Look at this sophistication," she says, stopping in front of a mannequin of Eleanora Duse in a Fortuny costume. "She's the most sophisticated in the whole show."

Some of the most beautiful dresses she has ever seen, such as the Charles Worth gowns worn by Louise Carnegie, wife of Andrew Carnegie, , are in the current show. So are some less attractive. "The 1880s was a repulsive costume period to a lot of people. Women then had such huge rear ends, which is offensive to women dieting today. Visitors are amused by it and say, 'Oh God, can you imagine looking like that?' " But Blum sees the beauty of the style because it reflects the time.

By comparison, Blum finds a lack of focus by today's designers in the current collections in Europe or New York. "There is no regard for discipline today, and fashion is a discipline. The current attitude toward authority today reflects no respect for policemen or teachers. Of course, it shows up in the lack of direction in clothes."

Blum has written five fashion reference books, and another, "Fashion in Godey's Ladies Book," is in progress. Her knowledge of costume brought her into a dispute in 1981 over the authenticity of a Georges de la Tour painting, "Le Tricheur." Blum proved the painting was not a fake through her familiarity with 17th century clothing and paintings of the period.

Blum has had a high standard for costume acquisitions at the Met. The ultimate test, she says: "Is it a work of art and does it represent its period? It's wonderful if it belonged to someone who was important historically, but that's a consideration way down the line." In saying that, she is answering those critics who say, "You are oh so snobbish at the Met" and "You don't have anything that belonged to poor people."

"Well, we don't collect paintings by Sunday painters, either . . . We always try to emulate our betters. If you get what the people who can afford the best wear, you know that along the line the fashion will sift down. Good artists influence other artists. It is the same kind of deal."

Everything's in place for her move. She has established a master's program in costume at the Metropolitan with New York University. A new lab in the Costume Institute will apply scientific study to costume, and experiments will be conducted to seek out new technological ways to restore and preserve costumes.

The costume collection at Kent State will have nowhere near the 47,000 pieces in the Met's collection. But Kent State has more than 3,000 pieces selected by the designer/manufacturer team of Jerry Silverman and Shannon Rodgers. "It's a jewel of a collection," says Blum, who has been spending part of the last few months in Ohio working with architects to lay out the new museum.

"I'm not going to miss New York because it's going to be so active there," says Blum.