Judith Blegen opened the Kennedy Center's new art song series in the Terrace Theater last night and in no time at all cast an unbreakable spell over the audience.

She is lovely to look at and a joy to listen to. Her light, lyric soprano floats some of the most ravishing soft notes to be heard these days, and she uses it with a technique that conquers the Mozart "Alleluia" with the same ease as the long lines of Pamina's "Ach, ich fu hls."

What's more, Blegen is a superb musician, having played the violin before she began to sing. In six songs by Brahms, several of them among the more rarely heard, she offered elegant phrasing and total projection of the meaning of the texts. She is, in other words, precisely the kind of artist Marta Istomin had in mind when planning these song recitals. Blegen also displayed an artistic generosity by singing on short notice in place of the ailing Swedish baritone, Hakan Hagegard.

One of the finer actresses now on the world's opera stages, the lovely soprano triumphed over a minor contretemps that arose during her singing of "Schlagende Herzen" by Richard Strauss. In a gesture with her right hand, a bracelet on her wrist got tangled up with the rhinestone decoration at the top of her nearly strapless gown, making it impossible for her to move her hand. It was a near thing for a moment, but with irresistible insouciance, Blegen managed the trapped wrist with fetching grace until the end of the song. Her audience could not have loved it all more.

While Mozart's "Alleluia," Bellini's "O quante volte," and Monica's scene from Menotti's "The Medium" stretch things a bit on a program of songs, each could be defended as an example of songwriting, and Blegen kept them within proper bounds.

Two old English songs led to the giddy raptures of Milhaud's settings of Ronsard which Blegen sang in perfect French and brilliant style. One matter in her singing could be corrected with ease: in soft passages words tend to disappear. From so distinctive an artist this is unnecessary. The encores were Solvejg's Song and Stravinsky's wordless Pastorale.

Pianist Martin Katz played with consummate art throughout the program.