Poor Blanche (Vivien Leigh), always hiding in the shadows so suitors won’t realize her age. She’s a woman obsessed with sex and terrified of death as it relates to her aging and dwindling beauty. She’s determined to believe that asserting her sexuality upon younger men will somehow render her immortal, and agelessly beautiful by proxy.
As such, Blanche is in love with darkness throughout A Streetcar Named Desire (1951). Even when she’s out practicing the art of seduction with Mitch (Karl Malden), she keeps to dimly-lit areas and shadowy corners. By design, nobody has seen her in the day, adding to her mystique. In the Kowalski house, she shades the light with a paper lantern, making Mitch assemble it on the fixture before turning on the bulb.
But Blanche is hiding more than the age lines on her face - showering herself in darkness is a means of hiding away from society - a society she knows she’s offended. She’s ashamed of her sexual behavior, her loss of dignity, and tucks herself away in the dark. She’s haunted by the loss of her husband to homosexuality, and the loss of her elite lifestyle at Belle Reve. Beautiful black and white cinematography works with both light’s literal and metaphoric uses in the film.
“We get another layer of meaning to this lights business when Blanche discusses her former husband, Allan. She describes falling in love as though 'you suddenly turned a blinding light on something that had always been half in shadow, that’s how it struck the world for me.' When she caught him with another man, later confronted him, and discovered his suicide, she claims that 'the searchlight which had been turned on the world was turned off again and never for one moment since has there been any light that’s stronger than this — kitchen — candle.'” - Shmoop
When Mitch finally realizes he's never seen Blanche vividly and forces her into the light, he tells her he doesn’t mind her age, only her deceitfulness. She responds by saying she meant no harm, and simply believes that magic, not reality, is the best representation of life. This is a clear indicator of the direction Blanche’s mental state is headed.
She also tells Mitch about her relationship with her late husband, which was like having the world bathed in a rich, white light. Ever since his death, through all the sexual partners she’s had, there has only been dim light. Brightness becomes parallel to her sexual innocence and vitality, while dim light is her aging reality.
The paper lantern becomes a metaphor for Blanche herself. She blocks the light of the world so she doesn’t have to face its realities. She can live on, pretending she’s young and beautiful and desirable, not the person she really is.
Blanche also associates light with poetry and music, which she speaks of as a means of contrasting herself against the brutish nature of Stanley (Marlon Brando), whom she classifies as socially beneath her.
“There has been some progress since then!” she says to Stella (Kim Hunter). “Such things as art—as poetry and music—such kinds of new light have come into the world since then!... In this dark march… don’t — don’t hang back with the brutes!"
She romanticizes the arts (as an English teacher), relating them to brightness and light. As with all the other light symbols in her existence, her teaching days are in the past. It’s one more way Tennessee Williams’ careful attention to word usage contrasts Blanche’s current position to her former life, and shines light on the madness she’s sinking into in the darkness.