Making a Legend More Macabre : THE BLACK DAHLIA<i> by James Ellroy (The Mysterious Press: $16.95; 336 pp.) </i>

On the morning of Jan. 15, 1947, the body of a nude young woman, severed at the waist, was found in a vacant lot on Norton Avenue.

This macabre discovery touched off a police investigation and a newspaper extravaganza that have not been equaled since. The unfortunate victim turned out to be Elizabeth Short, 22, a Hollywood tramp from Medford, Mass. Her killer has never been caught.

Perhaps the quality that gave this grisly case its flavor and kept it on the front pages for months was Elizabeth Short’s nickname. Because of her bouffant black hairdo, her friends had called her the Black Dahlia. Once the newspapers got wind of this sinister sobriquet, what had been a tawdry crime became an enduring classic.

In life, Elizabeth Short had never been more than an unlucky Hollywood hustler, pretty enough but no great beauty. In death, she became a legend. Many hapless suspects were questioned and reluctantly let go; many more came forth to confess, but the police had held back certain clinical details of the victim’s dismemberment, which the would-be confessors did not know and could not guess.

James Ellroy’s novel is not the first entertainment derived from this event, but it may be the most imaginative and bizarre.

The narrator is Bucky Bleichert, a cop and ex-prizefighter; his partner and friend is Lee Blanchard, also a cop and ex-prizefighter. Both seem to be in love with Kay Lake, ex-girlfriend of an imprisoned robber-pimp whom Blanchard busted. Bleichert won’t touch her because she is living with Blanchard, although they don’t have sex.

This strange menage a trois is made all the more peculiar by the pathological influence of the dead Black Dahlia. Blanchard is obsessed with finding her killer, because he feels guilty for the mutilation murder of a younger sister; Bleichert has an erotic fixation on the Black Dahlia, so that she intrudes on his sex life--when he’s making love to another woman, he sees her face.

Bleichert has a Dahlia-haunted affair with Madeleine Sprague, a young woman who was or was not a friend of the Dahlia’s, and may have had a lesbian encounter with her; he does not report her connection with the murdered girl, if indeed she had one.

As they did in the real case, the cops sift through the sordid underside of Hollywood, digging up casual friends of Elizabeth Short, tracing her to a pornographic ring in Tijuana. Blanchard turns up there, now obsessed not only with the Black Dahlia case but also with Kay Lake’s ex-boyfriend, who is out of prison and presumably bent on vengeance.

When Blanchard disappears, Bleichert ends up in Kay’s bed, but he still sees the Black Dahlia’s face. He goes crashing about on his own, and finally, through Madeleine, finds the murderer, who turns out to be as psychopathic and improbable as the real murderer would probably turn out to be.

For reasons that should not be divulged, the murderer is never revealed to the cops. That, at least, is in keeping with the facts.

The book is turgid with passion, violence and frustration; the dialogue is suitably obscene for a cop book, and the prose is riddled with such graphic synonyms as “eyeballed” for looked at.

One error needs correction: Ellroy refers to the old Daily News as a “Republican rag.” It was, on the contrary, a Democratic beacon.