Blanche is a holdover from the Old South. She has values and ideals that are inappropriate for the modern world. Her decline and fall are symbolic of the decline and fall of the aristocratic Old South which was so heavily dependent upon slave labor and so fatally wounded by defeat in the Civil War. She is an idealist and a romanticist. When she taught English her favorite poems must have been those of such unworldly poets as Keats, Shelley, Byron--and perhaps especially those of Tennyson.
Her interest in young boys shows that she is not concerned about carnality but about beauty and love. She is intended to contrast with her sister, who is married to a brute of a man who overwhelms her with his love-making and gets her pregnant. After the wild night in Scene Three, Stella makes love with Stanley and is blissffully exhausted in Scene Four. When Blanche expresses concern about their tempestuous relationship, Stella says:
But there are things that happen between a man and a woman in the dark--that sort of make everything else seem--unimportant.
A man like that is someone to go out with--once--twice--three times when the devil is iin you. But live with? Have a child by?
I have told you I love him.
Blanche doesn't want that kind of love and has probably never experienced it. She was once in love with a poetic young man who committed suicide, and she seems to be looking for that same kind of romantic lover who confines most of his passion to kissing, holding hands, and reciting poetry. When the paper boy come by while she is alone, she is fascinating by him. She says:
Young man! Young, young, young man! Has anyone ever told you that you look like a young Priince out of the Arabian Nights?
Unfortunately, this is the kind of romantic slush that Stanley Kowalski would despise and laugh at. She is scared to death of him because he rips her flimsy fantasy world to shreds.
Blanche tells Mitch:
I don't want realism. I want magic! Yes, yes, magic! I try to give that to people. I misrepresent things to them. I don't tell the truth. I tell what ought to be truth. And if that is sinful, then let me be damned for it.
Her interest in boys only symbolizes her totally unrealistic, impractical mentality, and her refusal to adapt to reality leads to her decline and fall. She might have survived longer if she had never come to live with her sister and her monster of a brother-in-law. She creates a nonnegotiable conflict because she so strongly disapproves of Stanley as being unworthy of women of her own and Stella's social class. She tries her best, using her remaining authority as older sister and representative of the Old Southern aristocracy, to alienate Stella from Stanley. Stanley may be uncouth--but he is not stupid. He sees that Blanche is threatening his entire world, trying in her sweet, insidious way to separate him from the woman he loves and from the baby who is about to be born.
The coming baby creates a "ticking clock." The apartment is already too small for three people; it will be impossibly overcrowded when there are four. Blanche knows full well that she will have to move out when the baby comes, and Stanley makes it quite plain that he will evict her when that happens. She is running out of options. She can't get another teaching job because she has shown too much interest in schoolboys. If she can't entice Mitch to marry her she will be destitute--but Stanley has poisoned Mitch against her.