Grandma Was a Back Bay Boho : NONFICTION : THE WHITE BLACKBIRD: A Life of the Painter Margarett Sargent by her granddaughter,<i> By Honor Moore (Little, Brown: $24.95; 416 pp.)</i>

Born in 1892 with a silver spoon in her mouth, she was so well-known that, as a child living in the patrician splendor of turn-of-the-century Boston, the post office once delivered a letter to her family’s Back Bay mansion addressed simply “Margarett Sargent, Boston.” Nearly 20 years after her death in 1978, however, only a few people recognize her name, while more confuse her with her illustrious fourth cousin, John Singer Sargent, the high-society portraitist whose lush paintings of the landed gentry formed the very standard against which this avant-garde painter and sculptor was rebelling.

In “The White Blackbird,” her granddaughter, Honor Moore, attempts to rehabilitate Sargent’s reputation and clarify the mystery that lies at the heart of her troubled career: Why in 1936, at the height of her reputation, did this renegade socialite, as flamboyant as another Bostonian enfant terrible, Isabella Stewart Gardner, stop dead in her tracks, abandon painting, fend off the importunate gallery directors eager to exhibit her work and turn instead to the relaxing pastime of horticulture, pottering around the tulip beds and the topiary gardens of her immense North Shore estate?

Rejecting the conservative expectations of her Boston Brahmin family, Sargent studied with some of America’s most famous artists, including Gutzon Borglum, the sculptor of Mt. Rushmore, and George Luks, an iconoclastic painter who, in conjunction with a band of artistic revolutionaries known as “the Eight,” broke the academic stranglehold of New York’s National Academy of Design on American art. Sargent was soon enlisted in the modernist cause as both a working artist and an unfailingly astute collector who--buying art by such luminaries as Picasso, Gauguin, Lachaise, and Calder--became one of the major conduits through which Americans were introduced to the work of the European avant-garde. During the 1920s she played an instrumental role in turning the Boston Art Club into a vibrant outpost of modernism. This bastion of experimental work regularly held its own Salon des Refuses in the middle of a culturally reactionary town that many progressive critics dismissed as a provincial backwater all but closed to the influences of the painters with whom Sargent often exhibited: Matisse, Rouault, Modigliani, Picasso and Braque.

When she wasn’t painting, sculpting, purchasing and hobnobbing with everyone from the writer Jane Bowles to actor Mickey Rooney, from music-hall celebrity Fanny Brice to poet Archibald MacLeish, she was juggling the contradictory demands of her sadly neglected children, her philandering financier husband, her magnificent gardens, her two palatial homes and her own tempestuous extramarital affairs with men and women. The strain of this complicated life took its toll and she was soon such an incorrigible alcoholic that she would fall asleep face-first in her plate in the dining room of the Ritz Carlton and her husband could smell whiskey in the teapot on her breakfast tray.

Not surprisingly, she was by all accounts a lousy mother (a label often applied to female artists)--so bad in fact that her son once claimed that if the house were on fire and his mother had to choose between rescuing a child or saving a Renoir, “the Renoir would win, hands down.”

Without warning in 1936, she threw in the towel, explaining cryptically to her family that her career as one of America’s most promising painters had become “too intense.” From that point forward, her life slowly unraveled into an endless succession of protracted holidays in detox clinics, luxurious sanitariums and bucolic psychiatric hospitals where, in the throes of depression, she was stuck in a straitjacket and strapped to a table while a doctor fitted a metal plate to her head and turned on the juice of the treatment of last resort, electroshock therapy. Over time, she became so disoriented that, when a psychiatrist once tested her mental competence by asking her how much change she would receive from $5 if she purchased one item for 25 cents and another for 50 cents, she hemmed and hawed until she contemptuously replied that she was an artist and had never been very good at figures.

Moore’s explanations of her grandmother’s artistic and emotional disintegration are the weakest part of an otherwise gripping book about a painter who has all but disappeared from the history books after occupying a preeminent position in the vanguard of revolutionaries who brought Montmartre to America, ushering in such movements as Cubism, Expressionism, Dadaism and Surrealism.

“The White Blackbird” is a typical example of hagiography, a saint’s legend in which Moore invents a martyred innocent victimized by the sexist drudgery of keeping an upper-class Boston household while preserving the fragile illusion of being the perfect mom, hostess and homemaker even as she was wrenched apart by her socially unacceptable ambitions of being an artist. Not only does this interpretation seem psychologically simplistic and even patronizing, especially in reference to this fiercely autonomous, privileged individual, but it portrays her misbehavior--her neglect of her children, her cruelty to her husband, her drunkenness--as incontrovertible evidence of her superiority.

Her faults were not, according to Moore, personal failings: They were symptoms of failings in the unfeeling world around her, in the heartless wilderness of Bostonian high society or in the dour and unsensual world of status-conscious New England, which, haunted by the ghosts of the Puritans, sought to force this sexually transgressive pioneer into a role for which she was pitiably miscast, that of a cloyingly sweet mother and homemaker.

Moore fails to do justice to her subject’s brilliance and complexity by drafting her into the modern feminist allegory, shaping the inchoate details of her unhappy life into a pat, reductive object lesson about a rebellious free spirit who struggled valiantly to wriggle out of the constricting social mores that oppressed her.