To truly understand the role of the Mexican woman selling flowers in "A Streetcar Named Desire", one must translate what she calls as she walks the streets.
The flower seller calls out "flores para los muertos". This means "flowers for the dead". Blanche has just had an argument with Mitch and he tells her that he cannot live with the truth of her past (she was a prostitute.) After Mitch leaves, Blanche stumbles to the door of Stanley's flat and sees the woman selling flowers.
The symbolism of this is two-fold. First, Blanche's language becomes indistinguishable- her words mirror the foreign tongue of the Mexican. This shows that she is no longer able to be understood, literally and figuratively.
Second, and perhaps the most profound, is the foreshadowing that is provided here. The Mexican woman is calling for people to buy flowers to show respect for their dead. This foreshadows Blanche's own figurative death. Mentally and physically, Blanche's life is over. Mentally, Blanche has lost it. Physically, she knows now that her youth, and chances for marital happiness, are gone.