Most major practitioners of jazz have been, and continue to be, Black musicians. In the field of classical music, however, the picture changes. Few Black performers have overcome all the barriers to a notable classical-music career. Singers, especially women, constitute the only exception.

Among instrumentalists, the one Black superstar is pianist Andre Watts (b. 1946), who burst on the music scene at 16 and has since fully merited his worldwide acclaim; he has been for some years one of the four or five finest pianists in the world.

But few realize that the plight of Black players in recent decades has been worse than it was several generations ago. Several past notable violinists include Jose White (1833-1920), who was a concerto soloist with the New York Philharmonic more than once in the 1870s; Joseph Douglass (1869-1935), grandson of the legendary Frederick Douglass and the first Black violinist to tour the United States as a recitalist; and Clarence Cameron White (1880-1960), who was active as a composer in addition to his concertizing.

Only in the past few years have there been signs that things are looking better for Black string players. Part of the evidence is a handful of recent recordings.

In the world of the violin we have Sanford Allen (b. 1939), who entered the Juilliard School of Music at 10, and by 20 was accomplished enough to play as a substitute violinist in the New York Philharmonic. In 1962 he became the first full-time Black player in that orchestra. Over the years he gave occasional recitals, and in 1977 he courageously decided to give up his guaranteed income from the Philharmonic in favor of a career as a freelancer and soloist.

Allen shows what he can do on the fourth of nine disks that Columbia Records issued in its Black Composers Series, a project that the firm unfortunately seems to have discontinued. With the Detroit Symphony Orchestra, led by Black conductor Paul Freeman, Allen gives a stunning performance of the Violin Concerto (1962) by Black composer Roque Cordero (b. 1917), a native of Panama who for some years has been professor of music at Illinois State University.

The concerto combines twelve-tone serial procedures with traditional structural designs. From a sonata-form first movement, it moves through a ternary slow movement to a rondo finale. He uses a 12-tone row, both forwards and backwards, but the piece is still easier to follow by ear than most serial works. Cordero's orchestration is so skillful that even the rambunctious finale never swamps the solo violin. Like its famous Alban Berg predecessor of 1936, this work is a rare masterpiece among serial violin concertos.

Having played the violin and viola in his youth, Cordero fashioned a highly idiomatic solo part. Nevertheless the writing twists fiendishly and Allen plays equal to every demand. His intonation is absolutely secure in huge leaps, and he always maintains control of his bowing arm. He negotiates all the double stops, harmonics, left-hand pizzicati and tricky rhythms without a hitch.

The one flaw is a 72-bar cut in the recapitulation of the first movement. Though the cut was sanctioned by the composer, it would have been preferable to present the work whole, since the concerto surely will not be recorded again soon. This aside, the performance is definitive; and the disk, filled out with Cordero's Eight Miniatures for Small Orchestra, won for Allen the Koussevitzky Recording Award from an international jury. But outlets for Allen's dazzling abilities will depend on the colorblindness of concert managers and audiences.

On the other hand, no past Black musicians carved notable careers as viola soloists. Enter one Marcus Thompson (b. 1946), who holds three degrees from Juilliard and has taught music at M.I.T. since 1973.

Thompson interspersed his academic duties with appearances in chamber-music concerts and as guest soloist with numerous orchestras, following his professional New York debut in 1968. He has performed such staples of the viola repertory as Berlioz' Harold in Italy, and in 1976 was soloist in a New York performance of the Viola Concerto by the late Harvard professor Walter Piston. Two years ago he gave the premiere of a work by a Black composer from Ghana, Samuel Johnson, with an orchestra led by Black conductor Karl Hampton Porter.

Early this year, Thompson was a winner in the National Black Music Competition at the Kennedy Center in Washington. As a result he and violinist Yehudi Menuhin are scheduled to perform the Mozart double concerto with the Chicago Symphony this coming season.

Thompson recently released a disk on which he plays three works in collaboration with the M.I.T. Symphony Orchestra, conducted by David Epstein. Thompson himself has provided insightful jacket notes on the music.

Side one is devoted to Paul Hindemith's Der Schwanendreher (1935), the third of his four pieces for viola and orchestra. The work is a three-movement concerto, which uses four German songs from the Renaissance for much of its thematic material, a feature that illustrates Hindemith's lifelong interest in early music.

Hindemith was also a professional viola soloist and gave the work's premiere, later recording the piece with Arthur Fiedler after he emigrated to America. The performance by Thompson and the M.I.T. forces is perfectly acceptable, but there is no reason to prefer it since the work is currently available in four other performances with professional orchestras.