What's a dry cleaner's nightmare? To be entrusted with the glitzy, glamorous "La Cage aux Folles" costumes -- the most expensive in town -- and ruin them hours before the next performance. Which is exactly what Blackstone Cleaners did not so long ago when its equipment turned the glittery sequins on six "La Cage" costumes a dingy gray.

"Too much steam," recounts Steven Bender, Blackstone's owner. "It was a disaster," he adds, noting that his company paid to have new sequins hand-sewn onto the outfits.

Other than that one bomb, though, Blackstone has performed well in the high-pressure, fast-paced world of theatrical costume cleaning.

With a clientele that includes the Kennedy Center, the National, Ford's and the Warner, Blackstone must routinely pick up the costumes once a week after an evening performance and return them by 3 p.m. the following day.

Some of the stars don't make the task easy. "Rex Harrison insists on a lot of starch," says Bender. "He wants us to make sure that his collars stand straight up."

Peter Marshall is another case. According to Bender, the star of "La Cage aux Folles" wants starch in his collars but not in his cuffs, a difficult request to comply with because starch is ordinarily added to the water. Auditions for Hexagon

So you think you're funny, can sing and dance and would like to hit the boards for a worthy cause? Here's your chance. Hexagon will hold auditions for its March revue, "Roast of the Town," from 10:30 a.m. to 6 p.m. Saturday and from 1:30 to 6 p.m. Sunday at St. John's Church, 3240 O St. NW. For more information call 642-9317.

Now in its 31st year, the annual Hexagon revue, which spoofs Washington politics and life styles with original music and comedy, has raised more than $1 million for local charities. This year's beneficiary will be The Hospital for Sick Children.

Guest performers who already have agreed to appear in at least one of the 19 performances include former senator Eugene McCarthy, former White House press secretary Ron Nessen, Rep. Bob Dornan and Redskin Kenny Jenkins. First Lady Nancy Reagan will serve as chairwoman for the March 15th Gala Performance, which will be followed by a black-tie dinner and dance at New South Hall on the Georgetown University campus to raise additional funds for the hospital.

Hexagon moves this year from the Trinity Theater to the larger and newly renovated theater of the Duke Ellington School of the Arts, making available 4,000 more tickets than in previous years to the traditionally sold-out month-long run. Tickets go on sale Feb. 24. Yiddish Theater

A black fedora worn in a Yiddish production of "Death of a Salesman," posters promoting the 1937 staging of "Money, Love and Shame" in Savannah, Ga., and letters, photographs and playbills from the collection of actress Molly Picon are but a sampling of more than 250 artifacts in the "Hooray for Yiddish Theater in America!" exhibit at the B'nai B'rith Klutznick Museum through August.

Though today amateur companies such as PerHift in Milwaukee and the Folksbiene in New York struggle to maintain regular seasons, Yiddish plays were once staged in dozens of theaters in Manhattan, the Bronx and Brooklyn, as well as in theaters across the Midwest, bringing fame to actor Boris Thomashefsky and Yiddish Art Theater founder Maurice Schwartz, among others.

Exhibit designer Chris White has incorporated theatrical props, special lighting and theater seats to create a nostalgic environment for the artifacts and text. For further information, call 857-6536. A Play About AIDS

Washington is still waiting for its first play that deals with the timely topic of AIDS. The New York Theatre Studio recently dropped its preliminary plans to mount a production here of "The Normal Heart," which focuses on the New York homosexual community's response to the fatal disease, after Center Stage announced it would open the play in Baltimore Jan. 24.

Jim Petosa, codirector of the New York Theatre Studio, said the group is "actively pursuing another option." The theater is considering commissioning a play that "further elaborates on where the AIDS issue is going," Petosa said, explaining that such a play would consider the political reaction to AIDS. Odds and Ends

Horizons begins its winter workshop series with "Introduction to Acting," an eight-session class beginning Feb. 3, and "Scene Study," a 10-session workshop starting Feb. 4. For further information call 342-7706 . . . The Omni Shoreham Hotel tonight offers two special performances of the musical comedy revue "Forbidden Broadway" -- an early show at 7 p.m. and a festive gala performance at 10 p.m., which climaxes with champagne toasts at midnight . . . Shaw's "Heartbreak House" opens Friday at the Source's Main Stage.