There still are people who listen to the Joni Mitchell albums "Blue" and "Court and Spark" and reminisce about the good old days and hownice it would be if Joni went back to that kind of music.

Joni's recent releases have met with solid resistance from all but die-hard fans andpeople whose interest in popular music is beyond average. The disenchantment began with "The Hissing of Summer Lawns" and rose with "Don Juan's Wreckless Daughter." Joni said she was incorporating jazz; some said she was producing junk.

Now Joni Mitchell has taken a giant step. She has most certainly incorporated jazz. She is most certainly not producing junk, though this album may get less radio play than any other in hercareer.Mitchell's new "Mingus" is a significant work.

It is significant for reasons that go beyond its music. The story behind "Mingus" is that legendary jazz bassist Charles Mingus, dying of Lou Gehrig's disease, decided to compose several melodies that Mitchell would put words to.

This act seemed to contradict just about everything in Mingus' character. He was known as a bit of a bigot; that he would ask a white singer with little jazz experience to work with him on his final compositions washard to believe. Mitchell had never written lyrics for someone else's music, and Mingus' method was not your basic 32 bars.

As word of the project spread, the question became who would play the record and who would buy it. Rock people cringed at the thought of a pure jazz album from a rock performer; jazz people weren't thrilled with the idea either. Mingus and Mitchell thumbed their noses at both.

That is some gesture ina music world where Kiss enters the charts at No. 17 and "Mingus" has five of the last Mingus compositions, and that alone makes it important. It also has Mitchell's interpretation of "Goodbye Porkpie Hat" and five"Raps" - short statements by Mingus which are neither maudlin nor self-indulgent.

Mitchell's vocal work, which no doubt will be scrutinized byevery pious jazz afficionado in the western hemisphere, is interesting and effective in most places. But she is not a true jazz vocalist.

On " Mingus," Mitchell does display some of her most inspired singing since "Court and Spark" and her way of flowing through octaves without a breakis distinctly non-pop. In fact, some of her phrasing is so elongated that you can lose the rhythm of a peice until an instrumental break pullsit back together.

Once again, Bassist Jaco Pastorius is front and center, but this time he sounds slightly more subdued and less chilling. Whether this is the nature of the compositions or Pastorius' own sentimentfor Mingus, the effect is warmer. Mitchell shows she's still capable of sending chills up your spine, a la "Don Juan's Wreckless Daughter," intunes like "The Wolf That Lives in Lindsey" (Mitchell was responsible for all the titles except "Goodbye Porkpie Hat.") There, her guitar hacking coupled with the sound of wolves baying makes you wonder if you're listening to the soundtrack of "Prophecy."

In general, though, Mitchell and her band: Pastorius; Wayne Shorter (soprano sax); Herbie Hancock (electric piano); Peter Erskine (drums); Don Alias (congas); and Emil Richards (percussion) do justice to Mingus and to themselves. Some of Mitchell's lyrics are suspect. In "Goodbye Porkpie Hat" she evokes with strident authority a period that she could only have read about. But this album is conceptually unique and some problems must be expected.

Mitchell is scheduled to appear in the Washington area in August, most likely with Weather Report (if you substitute Joe Zawinal for Herbie Hancock, her album band is Weather Report) and it will be interesting to see who comes. It may turn out that with the release of "Mingus," Joni Mitchell has succeeded in alienating all of her potential audiences. No matter. "Mingus" lives. And that was the idea all along. CAPTION: Picture, JONI MITCHELL; SHE'S VENTURING INTO NEW - AND DICEY - TERRITORY WITH THE ALBUM "MINGUS."