Light and shadow are a part of Williams's theme of reality versus fantasy in A Streetcar Named Desire. He highlights the difference between them, using their contrast theatrically and thematically.
Light represents truth and the reality of Blanche's past. She tries to hide from the light, such as by hanging the paper lantern over the light bulb in Stella and Stanley's house. She uses shadow to hide her appearance from Mitch, choosing only to go out with him at night and in locations that are not so well-lit. Blanche prefers to stay in the shadows. She says,
I like it dark. The dark is comforting to me.
She also refers to light when talking about her first love, her husband Allen:
He was a boy, just a boy, when I was a very young girl. When I was sixteen, I made the discovery—love. All at once and much, much too completely. It was like you suddenly turned a blinding light on something that had always been half in shadow, that's how it struck the world for me. But I was unlucky. Deluded .... And then 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 …
The reference to light and darkness represents Blanche's emotions.
When Stanley overtakes Blanche, Williams gives us this stage direction:
He crosses to dressing table and seizes the paper lantern, tearing it off the light bulb, and extends it towards her. She cries out as if the lantern was herself.
This represents his overpowering of her, and that he has won in their emotional struggle (in addition to the obvious physical one).
Additionally, Stella talks about darkness when discussing Stanley. Blanche can't understand why her sister would go back to a man that has hit her, but Stella explains,
But there are things that happen between a man and a woman in the dark—that sort of make everything else seem—unimportant.
Characters are able to hide in the shadows.