Cornball comedy, irreverant musical, and a diva on the ropes

From mid-Manhattan to Greenwich Village, several Off Broadway musicals have opened - and sometimes quickly closed - in recent weeks. Subjects have ranged from biblical musings to the New York singles scene, and from pseudo-biography to various aspects of life in these United States. Two newcomers still current at this writing are ''Blue Plate Special'' and ''Tallulah.'' Blue Plate Special. Musical comedy by Tom Edwards (book), Harris Wheeler (music) , and Mary L. Fisher (lyrics). Directed by Art Wolff.

Home cooking, cornball humor, and a dash of social significance furnish the principal ingredients of the folksy country musical at the Manhattan Theatre Club's Upstage. ''Blue Plate Special'' spoofs a Tennessee-style soap opera as it spins a tale about Della Juracko (Gretchen Cryer), her mixed-up relationships, and the threatened future of her diner.

When not coping with assorted crises, Della and her friends break into song, to the accompaniment of the diner jukebox (a cooperative offstage combo led by Jimmy Roberts at the keyboards). The bouncy score is by composer Harris Wheeler and lyricist Mary L. Fisher.

Librettist Tom Edwards imagines that the two acts of ''Blue Plate Special'' are two installments of Episode 41 in the continuing daytime drama broadcast from Morning Glory Mountain, Tenn. Crime, punishment, redemption, mystery, rivalry, and reconciliation are the stuff of this broad lampoon.

One of the sight gags involves the use of a small dummy figure to portray a three-year-old murder suspect. ''Blue Plate Special'' is that zany. A somber note intrudes with the discovery that Morning Glory Mountain has been fatally polluted by illegal nuclear waste dumping. It's curtains for Della's diner.

An engaging cast of singing and dancing actors makes the most of the preposterous scenario, even when the jokes are overworked and the nonsense overextended. Besides Miss Cryer's durably good-natured Della, the company includes Tina Johnson as her daughter, a would-be pop singer; Gordon Paddison as the daughter's slick-haired sheriff husband; Ron Holgate and David Strathairn as Della's spouses, past and present; and Mary Gordon Murray as a former Nashville star who yearns for a comeback now that she is out of prison.

The performers occasionally kick up their heels and leap atop the tables of David Jenkins's cheerful eatery setting. Douglas Norwick devised the choreography. ''Blue Plate Special'' has costumes by David Murin and lighting by Arden Fingerhut. From an upper shelf perch, a lighted photo of Elvis Presley looks down on the proceedings with a bemused smile. And well he might.

''Blue Plate Special'' runs through Nov. 27. Tallulah. Musical by Tony Lang (book), Arthur Siegel (music), and Mae Richard (lyrics). Directed and choreographed by David Holdgrive. Starring Helen Gallagher.

The new musical starring Helen Gallagher may well have been affectionately intended as a tribute to Tallulah Bankhead. Miss Gallagher, an ingratiating personality and first-rate performer, makes la Bankhead a likable, wisecracking hussy. But the emphasis is almost entirely on the late star's disorderly personal life, indiscretions, promiscuity, and overindulgences. At her best, Tallulah Bankhead was one of the finest actresses of her time. ''Tallulah'' trivializes its subject with the kind of treatment ordinarily associated with supermarket scandal sheets.

Tony Lang's book comprises an autobiographical reminiscence in which Miss Bankhead skims over such matters as her decorous Deep South origins, adoption by the wits of the Algonquin round table, and her indecorous misbehavior on both sides of the Atlantic. According to Mr. Lang, parental objections (in the person of Russell Nype as disapproving Congressman Bankhead) dogged his daughter's career and led to their stormy parting in London.

The marvelous Miss Gallagher is good for all the high kicks and high spirits required of her starring role. Among the more enjoyable comic numbers in the campy parody is a Tallulah stricture on the New York subways, ''Don't Ever Book a Trip on the IRT.'' Although largely wasted, Mr. Nype has some appealing moments in ''When I Dance for You'' (a duet with Miss Gallagher) and in a touching reprise of the title song.

The brisk-paced accompaniments are in the hands of a two-piano-and-percussion trio led by Bruce W. Coyle, music director and arranger. ''Tallulah'' was designed by John Falabella (scenery and costumes), Neil Bieff (Miss Gallagher's shimmering wardrobe), and Ken Billington (lighting). David Holdgrive staged and choreographed the production.