A few weeks ago Patti Labelle, the most exciting rhythm and blues singer around, ran into Ella Fitzgerald, the venerated jazz doyenne, at an airport. The two women talked about disco, its double-edged effect on black artists, its ability to bring fame and simultaneously to squelch the art of black song.

Fitzgerald was as pessimistic as Labelle. "She was very concerned, expressing fear about the dearth of material and opportunities for the pure vocalist. She indicated that she had been giving some thought to how she fitted into this picture," said Labelle, worry lines settling across her bubble of a brown face. "She was also very gracious and said she admired me for not having given up, yet."

Patti Labelle's appraisal is frank and arises from her dismay that the non-disco female vocalist, as a species, is endangered. It is a melancholy thought and her voice picked up the tone. She sounded angry even though her mood was light. Washington, after all, is a good city for her. She opened a three-night gig at Constitution Hall last night and is to sing the National Anthem at the Washington Bullets game Sunday. "The response here always gives me some new energy," she said.

Yet still she frowns. Recently she has had the same conversation about disco's infringement and dilution with Phyllis Hyman, an up-and-coming singer, and music executives, as well as Fitzgerald. The number of vocalists who don't bend to disco's 125 beats a minute - and its sweet cash-register rings - is shrinking.

Also, any discussion of disco points out the ambiguities of her own status and success. She has been stuck by some with a disco label ever since she was the lead singer on Labella. And while she has enjoyed the popularity with disco followers, she rejects the classification.

"I am not a disco artist," said Labelle yesterday as she gulped some creamed coffee to soothe the raw edges of her voice.

The voice began to rise: "I never could be a disco artist; some of disco is demeaning, just music with no sense, just a beat. The artists really don't have to sing." She paused, obviously searching for a good point, slightly uncomfortable that she sounded like a tirading critic.

"I do appreciate disco for helping people come back.

"Like Peaches and Herb. But you see, they did their disco number, then followed it with a ballad. When I did 'Music Is My Way of Life,' [an uptempo hit from her new album] I didn't record it as disco. It's a nice song, with a nice beat," said Labelle.

"But I don't think it's fair that Cher - who is a friend of mine, but I'm getting really bitter about this - that Cher, Rod Stewart and the Blues Brothers can get the kind of play they do on black stations, but it takes quadruple platinum for us to be on the pop stations. It's getting unbearable."

These observations don't flow from any personal disappointments.Labelle is successful. Her last tour with bad-boy comedian Richard Pryor was a sell-out; her third album has sold 200,000 copies in a few weeks. Now in her third renaissance for the public, she's very satisfied. For 28 months now she has been a single act, severed from Labelle, a trio of fantasy and funk that was an outrageous 1970s recycling of Patti Labelle and the Blue-Bells, a '60s group of bop and tears.

"It's not as hard as I thought it would be," said Labelle, referring to her latest phase. For 17 years she had performed with Nona Hendryx and Sarah Dash. "I think we were all pretty brave to go out on our own. And the public has helped; every bit of encouragement has strengthened me. And now I know I am appreciated."

High on the list of reasons for that popularity is the breadth and energy she now brings to the packed-but-slumbering field of female vocalists. Diana Ross remains capable of funk, but her Las Vegas show-business approach is too structured; Natalie Cole hasn't yet found her niche; Melba Moore maintains an astonishing range but has slipped into disco; and Gladys Knight, Mavis Staples, Esther Phillips and Roberta Flack remain reliable but scarce. Valerie Simpson, a major influence in rhythm and blues, is a powerful force even within the context of Nick Ashford.

What Labelle offers, more than the others, is an astounding variation on a theme, ranging vocally from funk to ballad, ranging emotionally from a good pillow cry to a spirited church shout. "What I think I am achieving is a more settled, more casual, natural sound. It's even reflected in the show - a clean stage, no props, soft lighting, silk jeans and blouses," said Labelle.

Yet for all her years in show business, for all the peaks and lows, Labelle still craves star status.

"When I got off at the train station yesterday, every other person stopped me. I began to wonder if they have pictures of me in their bedrooms. I really looked like a dog. But when I become a household word, then I am a star, like Donna Summer, Barbra Streisand and the Carters."

The Carters? "Yes - Jimmy, Billy," she answered. "When I am that well known, it will make me feel I have gotten some due." CAPTION: Picture, Patti Labelle, by Joe Heiberger-The Washington Post.