At an abandoned mattress factory in east Los Angeles, Laurel Entertainment has established an unconventional studio for a low-budget but highly successful syndicated television series, ''Tales From the Darkside.''

In a recently filmed episode, ''Black Widows,'' Margaret O'Brien, who once played Judy Garland's kid sister in ''Meet Me in St. Louis,'' was cast as a spider lady grooming her daughter, played by Theresa Saldana, to fulfill the destiny of her venomous species. ''She wants to marry a thin man, so I try to fatten him up,'' Miss O'Brien explained. The unwitting victim is well fed in preparation for his fatal wedding night. ''Needless to say,'' Miss O'Brien observed, ''there's a bit of humor in the episode.'' Indeed, on the coffee table of the mobile home where the two spider ladies reside, a copy of Christina Crawford's novel ''Black Widow'' is prominently displayed.

''Tales From the Darkside,'' which is broadcast on approximately 125 local television stations including WPIX in New York (where it is shown Saturdays at 11 P.M.), has, without much fanfare, succeeded where the networks' expensive, heavily promoted suspense anthology series have faltered. NBC's ''Alfred Hitchcock Presents'' has been canceled. Steven Spielberg's ''Amazing Stories'' was bought with a two-year commitment last fall, so it will be returning to NBC next season in a new time slot, despite disappointing ratings. ''The Twilight Zone'' has also been renewed, chiefly...