Blanche makes a big deal out of never being seen in direct light. She uses a paper lantern like a shield to block out the strong light of the naked bulb in the Kowalski apartment. The obvious conclusion is that she’s getting older and doesn’t want anyone – particularly Mitch – to see that she’s no longer a girl of sixteen. Actually, Mitch says it best: "I don’t think I ever seen you in the light. That’s a fact! [..] You never want to go out in the afternoon. […] You never want to go out till after six and then it’s always some place that’s not lighted much. […] What it means is I’ve never had a real good look at you" (9.28-36).
Of course, if you want to get fancy, you might argue that Blanche is hiding more than just the fine lines around her mouth. We know that she’s ashamed of her behavior in Laurel and desperate to hide her past from Mitch. Is it possible that she’s manifesting this desire to hide in a physical way? Probably.
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" (6.120). 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…" (6.120).
In short, what she’s saying is that being in love illuminated the world for her. When her husband died, the world was in darkness again. What does this have to do with the paper lantern? Plenty. What we see now is that shielding the harsh light isn’t just about blocking Blanche from the plain view of the world – it’s also about blocking the world from Blanche’s eyes. She doesn’t want to see it. She doesn’t want to deal with reality. Does that sound like a Major Point in Blanche’s character? Good, because it is. (Read her "Character Analysis" for more.)
Blanche also uses light imagery to describe the benefits of poetry, music, and art – in contrast to what she considers to be Stanley’s primitive nature. She tells Stella, "There has been some progress since then! 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!" (4.118). It’s fitting that things like art and poetry are described the same way as love for Blanche – as forms of light penetrating the darkness of the world. Remember, she’s an English teacher, and her idea of love is an incredibly romanticized, poetic, artful one.
The last prong of this light imagery has to do with Stanley. We couldn’t help but notice that he describes the sex with his wife as "having them colored lights going" (8.55). He uses this phrasing twice, actually, which should draw your attention. It’s important that while light was a form of love for Blanche, it’s innuendo for sex for Stanley. It’s also important that the lights are colored, which is in contrast to Blanche (whose name means "white"). See "Tools of Characterization" for more discussion of this color business.