Tennessee Williams actually uses light reversely by playing with shadows in several scenes. From the moment she "moves in" to the Kowalski household, Blanche immediately tries to use shadows to help her create the fantasy world she prefers to live in by covering the naked lightbulb with a pretty Chinese paper lantern, thus masking the harsh reality the naked lightbulb displays/emphasizes. This reflects her struggle with reality and fantasy. This paper lantern and its effects are literally ripped away by Mitch who she tried to seduce with her beauty when he rips the paper lantern off the bulb, stating "I've never had a real good look at you, Blanche. Let's turn the light on here" (Scene 8). At this point, he is figuratively ripping her fantasy world away and is using the light to show her for what she really is: a woman with a dark sexual past. The Chinse paper lantern again comes up at the end as Blanche is being taken out by the medics from the asylum when Stanley asks if she wants to take it with her. This suggests that Stanley is offering what is left of her fantasy world to her, knowing she has completely lost her mind at this point and can no longer discern between fantasy and reality. Finally, the shadows near the end of the prostitute, the drunk, and the police officer also show the "shadows" of her past which she is trying to keep hidden and out of the light.
The most vivid example which comes to mind is when Stanley snatches off the paper around Blanche's lightbulb which makes her paper lantern. Up to this point she has been "softening up" certain hard realities of life (her signs of aging, dependance upon others, troubled past, alcoholism) with a hide-and-seek game nuanced in light and shadow. Stanley has had enough of his sister-in-law's airs and wants to tear away her mask to make her see herself as she really is. This taking by force foreshadows the rape scene to follow...
Stanley may seem cruel, but Blanche's forms of escapism totally alienate her in the end. Note that it is Stella who cannot accept reality in the end (her husband did indeed rape her sister!), and it is Blanche who is taken away and presumed insane.
Light is one of the symbols used by Tennessee Williams in A Streetcar Named Desire. Blanche places the paper lantern around the lightbulb in order to hide herself in the darkness for other not to see her, especially Mitch. She is aware that her beauty is fading away as she is growing old and feels insecure about it. She thought that Mitch would not accept her due to the fact that she is older than him. Thus, the light represents the hiding of Blanche's fading beauty.