MR. JUSTICE AND MRS. BLACK; The Memoirs of Hugo L. Black and Elizabeth Black. Random House. 354 pp. $22.95.

FOR ALL HIS courtliness, there was something essentially inscrutable about Hugo Lafayette Black, for 34 years (1937-71) associate justice of the United States Supreme Court. We have had, as yet, no book on "the enigma of Hugo Black." But by comparison, Felix Frankfurter (about whom just such a title was recently published) was a figure of perfect transparency.

If anyone knew Black well, it was Elizabeth Seay DeMeritte, also an Alabamian, who became the justice's secretary in March 1956 and, some 16 months later, his second wife. (The first Mrs. Black had died in 1951.)

Elizabeth Black's interesting but discreet diary, covering the years 1964-71, is the portrait of a near idyllic relationship. Fond memory is tinged, here and there, with inside-the-Court gossip. It is never malicious (more's the pity, some would say). At one point, she speaks of "the raw, naked force of Hugo's intellect and will," a force with which many political and judicial foes had become acquainted. But there is little persistent exploration of the angularities of Black's character.

This amplification of the record is, in other words, useful but not especially startling.

When the abrupt onset of circulatory illness drove Black to retire in September 1971 -- he had been playing tennis a few weeks earlier, and would live only a few days longer -- he had served to a greater age than all but two earlier justices: Roger Taney, Marshall's successor, had sat to age 87; Oliver Wendell Holmes Jr. to 90. But neither Taney nor Holmes, and few others, left so telling a personal stamp on the jurisprudence of an age.

At the time of his appointment -- Black had been for about a decade senator from Alabama, and as a staunch New Dealer became FDR's first appointee -- he was patronized by some jurists and journalists as a naif. Black, largely self-taught, pursued his own intellectual pilgrimage, marching to his own drummer. He soon set himself to overturn the "natural law" school of constitutional interpretation which, with his penetrating mind, he detected at the heart of an overweening judiciary.

In one or another guise, and in the service of various political doctrines, that mode of judicial construction encouraged judges to construe the Constitution by the measure of their own consciences. Against this judicial subjectivism, Black set his hand. Eventually, in the Warren Court years, he recruited a majority of the justices to his views, for a time. His was a severely, some would say dogmatically, positivist view of constitutionality. The Constitution meant what it said and said what it meant -- no more and no less. It was not for judges to embroider it. The enemy was what Black liked to call the "shocks-the-conscience" school of jurisprudence -- the mocking phrase was that of his rival Felix Frankfurter.

Yet it was not clear that the implications of this drastic positivism were widely or easily grasped. Elizabeth Black's diary describes amusing scenes in the justice's study in Alexandria, where he would strive to instill in his "boys" (law clerks) the subtleties of his austere doctrine. Certain journalistic phrases (associating Black, for instance, with a belief in "absolutes in the Bill of Rights") probably did more to obscure than clarify his views. And in interpreting statutes, as distinguished from the Constitution itself, Black could be as subjective as the next judge.

ELIZABETH BLACK was sufficiently steeped in the justice's judicial predilections. But reading her diary, one suspects that they were of secondary interest. Hugo Black's outlook interested her because it was his, but her diary (unlike, say, Frankfurter's) is no chronicle of high judicial-political gossip. It is a personal record of what the justices and their wives did and said (and sang!) after hours, of the Blacks' travels to Alabama or Florida for visits with friends and relatives, and always of the heavy regimen of tennis that Black's vigorous physical life demanded.

Nuggets do turn up. We learn, for instance, that Black vigorously disapproved of the "conditional" retirement by which Chief Justice Warren sought to stay on pending the confirmation of a successor. This was widely viewed as a political maneuver, designed to forestall the appointment of a new chief justice by Richard Nixon, then (1968) the probable GOP nominee. It collapsed in failure when LBJ's nomination of Justice Abe Fortas was withdrawn at Fortas' own request, after the Senate refused to end a filibuster against his appointment.

There are colorful personal touches as well. It is not surprising that Justice Black worked hard. Most justices do. It is new to discover the extent to which judicial worries kept him wakeful. Sometimes a slug or two of bourbon and branch water would be necessary to settle him down in the wee hours. But in the eyes of an adoring wife, Black could do no wrong -- not even when his nighthawk habits kept her awake.

Mrs. Black reports some of Hugo Black's random comments on the controversial events of his past. She dutifully records his "explanation" of the controversial (pre-Senate days) membership in the Ku Klux Klan: That you couldn't expect to win a jury trial otherwise. She records the justice's fancy, as it must be called, that but for the Klan episode he might have been the Democrats' vice presidential nominee in 1944. These oddments of history should be treated with caution, not because there is doubt of Mrs. Black's accuracy as a witness but because memory can be a self-serving instrument. There is, for instance, no independent evidence -- none known to this reviewer, anyway -- that FDR seriously considered Black for the vice presidential nomination in 1944.

One sad episode needs mentioning. Elizabeth Black witnessed the burning of Justice Black's conference notes in 1971. These are the notes justices keep on cases, discussions and votes in the exclusive week conclave in which they decide cases. Black's conference notes, covering several eras of modern Court history, would be an invaluable archive. But the justice had become obsessed with the notion that eyewitness sources aid and abet the willful distortions of historians. He was determined not to help them. Accordingly, from his death bed at Bethesda Naval Hospital, he commanded his son to burn the notes, and Hugo Black Jr. reluctantly complied. An Old Town Alexandria neighbor complained that the ashes from the Blacks' fireplace were drifting into her swimming pool. The burning was moved, but it continued. Thus unfolded what Mrs. Black calls "Operation Frustrate-the-Historians," an appalling crime against history. The justice, though far from senile, was not in the fullest possession of his judgment, and one wishes a deathbed whim had been, for once, defied.

Along with the diaries, Mrs. Black has included here the fragmentary "autobiography" that Hugo Black began writing not long before his death. Its most valuable portions deal with his Army career during World War I. It is otherwise sketchy.

Like so much else that has been written about Hugo Black, this book is tantalizing. He was probably the most influential jurist of the past half century. But he was a tissue of incongruities, of which the Klansman/libertarian anomaly is only the most familiar. Elizabeth Black's book adds, readably and pleasantly, to the record. But it is also a teasing glimpse of unexplored depths.