AT A BLACK tie dinner last week, former ambassador Walter Annenberg shouted across the room to New York designer Bill Blass, "I'd like to be your partner." For a moment Blass thought Annenberg was asking him to dance. Then he realized Annenberg was acknowledging that many of the women in the room that night were wearing Bill Blass clothes. And they had been wearing them under their sables during the day. If not Bill's, other identifiable, expensive designer creations.

Dressing-to-the-hilt has come to Washington with this inauguration. And because these new women in town will be part of the fashion scene here for the next four years, it is sure to have an impact on the way Washington women will dress.These women will wear expensive clothes, lavish jewels, gorgeous quality furs, entertain extravagantly in Washington just as they have been doing in California. They are rich, and they have no hangups about showing it off.

The trend to dressing up was obvious all over Washington this past week, from the Smithsonian to the Capital Centre, in the Jockey Club and the F Street Club. When they return they may not be so visible to the public as they ride around town in limousines and party at the White House, private homes and swanky restaurants. But they will be here.

It started before the new administration came in and before the sable set put their first gold minaudiere on a white damask cloth here. Bill Blass's couture business, his most pricey stuff, for example, is $1 million ahead of last year, and some of that is Washington business. He's not an exception. Expensive clothes, top quality shoes, luxurious furs are selling well everywhere, Washington included. After years of dressing down, women are wanting to dress up again.

"People are tired of being sloppy," says Eugenia Sheppard, longtime fashion critic and columnist. "People are finding it feels good and looks good to dress up again."

It was obvious at the inaugural balls and even at the Capitol Center that women dressed up at the level they could afford, from polyester crepe to silk taffetta. "Everyone made an enormous effort," commented Blass.

The first lady set the tone for it all with a lady-like wardrobe of expensive, tasteful clothes, the kind of stuff that put her on the best-dressed list in the late 1960s. The regal quality to her selections -- the inaugural gown with a train, the red suit worn with matching red halo hat, or crown, (or as someone described it, a centerpiece) was hard to miss. Whether it was her choice to look regal, was well received. "People really like the idea of seeing their first lady all dressed up," says Blass who adds, "In spite of enormous inflation, people seem to have a great desire to live better."

"By setting a high standard in the White House, I think it will make us all try harder," said Val Cook, vice president of Saks-Jandel.

For the past few years women here would defer questions about their clothes by offering, when asked who's dress they were wearing, "It's mine." "It is nice not to be ashamed of what you are wearing," said Betty Lou Ourisman at the Capital Centre last week. She was wearing a Galanos, she said.

The California sable set doesn't necessarily spend more on each outfit. They own more of them, love to be photographed in them and proudly announce the name of the designer, who is likely to be a friend. Certainly they've lunched together. They prefer sable because of its lightness, but also own other furs and usually have a fur that fills most occasions.

They bought lots of new clothes for the inaugural. "They had been seen wearing their clothes from this falll and ordered things in new fabrics," says Blass. Although they buy things at full price from L. Magnin or Amelia Gray part of the package is that they deal directly with Blass, who guides them through their big purchases, then joins them for lunch or dinner.

Blass is the hands-down winner of this crowd, followed not even closely by Oscar de la Renta, Galanos, Dior. Blass's clothes are showcase clothes, not glitzy show-off clothes. Perfect for the country club. Never exaggerated. Ruffles instead of sequins. "Elegance not flash," says Aniko Gaal of Garfinckel's after watching the new arrivals at the F Street Club, the gala and the inaugural ball. Blass' best moment this week -- the black-velvet-top, satin-skirted gown Mrs. Reagan wore to the gala and looked well in seated or standing. "Not bad," Blass said modestly the morning after. She didn't need a sequin."

"Californians are more dressed up and more perfectly matched," says one Georgetown socialite. "In Washington we do things more quietly, more individually. Maybe its our Puritan heritage," she said.

For every evening occasion the sable set wore a different long gown, even it if was inappropriate, as at the Capital Centre. You rarely saw the Californians in pants for daytime or evening functions. "We wear pants very casually," explained Maureen Reagan, who was wearing an Anne Klein suede skirt, tweed jacket and satin blouse to a Georgetown brunch. "We'd never wear pants to business or to a very dressy party," she said.

Nancy Reagan got high marks for her inaugural wardrobe, with criticism by some for the Day-Glo red of her Adolfo coat costume. The red almost jumped out of the television screen and was very bright against the backdrop of all the Reagan family in gray, even with competition from Toni Hatfield's red coat.

Mrs. Reagan may have done for the halo hat what Jackie Kennedy Onassis did for the pillbox. The open-crown, braided fabric style serves to decorate the hair without disturbing it. Lots of people are wearing hats these days, though mostly the soft, pull-down variety. But kids in New York have been checking out the thrift shops for funny little hats from the 1940s and '50s to wear with their clothes.

Other than the hat, the impact will be more general. Soft knitted suits in the board room in place of blazers. Bare shoulder evening dresses. Good jewelry. Mrs. Reagan's way of dressing is one that can be understood, admired and copied by lots of women.

Julius Bengtsson changed Mrs. Reagan's hair for the inaugural ball to a style pulled back and decorated at the back with a hairpiece, a style that has proved controversial. "It wasn't easy with her short hair," said Julius, "but I found a way to do it with clips and things."

Mrs. Reagan's New York hairdresser, Marc, invited hairdresser Julius for drinks at the Jockey Club. Julius is the first lady's California hairdresser and stayed at Blair House with the Reagans during the inaugural warm-up. Marc sees Mrs. Reagan in New York, says he expects to be invited down from New York to do Mrs. Reagan's hair for special occasions. "I told Julius I understood perfectly why she had chosen him to do her hair this time since he had taken care of her for many more years than I, and does her color" Marc said. He doesn't mind sharing Mrs. Reagan? "It's always good for a woman to have two lovers," he said.

The taxi driver watched the lady struggle to get out of the back of his cab in her huge skirted taffetta dress and cowboy hat. "This city is a goddam costume party," he said to his next passenger.

To him it's costume, to the Texans at the Texas State Society "black tie and boots" party it's the way they like to dress up. It looked, sometimes, like fashion done up by a window dresser, pushed to the extremes of display. But not true of all the Texas young and old, who other times during the festivities were wearing the pretty ruffled dresses of Blass and others. The Dallas crowd wore the more conservative style, the Houston-ites the more daring.

There was the man with the rattlesnake bow tie, complete with rattlers, and a rattlesnake cummerbund. And Rubalee Ball, who shares oil properties with her ex-husband, in a floor-length mink with huge collar and hem in fox, and Frank Allison, the architect and developer in Luv Ya Blue Stetson and matching lizard boots and his wife in white mink to the floor. (He has to turn on the air conditioner in the car so his wife won't feel too hot in her coat.) And Dot Vannerson who brought eight Oscar de la Renta Hapsburg-style embroidered dresses for the inaugural, and eight cowboy hats by Susie Creamcheese trimmed in sequins to match the dresses.

A number of this new crowd showed their loyalty to their designers and their country with their inaugural wardrobe. Sally Schoenseilder, who has a sheep ranch next to the Reagans brought her 12 Alfred Fiandaca gowns to be sure she'd have the right one, and cards with the designer's name in case someone asked. And Jeannette Longoria showed her colors with a red Saint Laurent, a white Valentino and a blue Dior gown.