The street vendor who makes an appearance at the end of scene 9 of A Streetcar Named Desire is only on stage for a very brief period of time, but her role nonetheless has great significance. An old lady dressed in black, she sells flowers for the dead, calling out in Spanish as she walks along the street.

The appearance of the flower vendor comes at a pivotal moment in the play. Mitch has just found out about Blanche's sordid past and he can't live with the truth of it. He always thought that she was a fine, upstanding Southern lady, the very epitome of respectability. But now that he knows what she's really like, he wants nothing more to do with her.

It's right after Blanche has an argument with Mitch that she sees the old lady selling flowers for the dead. In symbolic terms, Blanche is foreseeing her own figurative death, when, after suffering a serious mental breakdown, she will end up being carted off to an institution.

The fact that the old flower vendor is speaking a foreign language also highlights the fact that Blanche and Mitch cannot communicate meaningfully with each other, knowing that the lurid truth of Blanche's background has been revealed.

During Scene 9, Mitch confronts Blanche with her deceptions, and she finally tells him the truth about herself. As a backdrop to the scene, a blind Mexican woman selling "gaudy tin flowers that lower-class Mexicans display at funerals" continually sings out in Spanish, "Flowers, flowers, flowers for the dead." Before the woman appears, Blanche has already been reliving Allan's death in her mind, unable to get the polka tune out of her head that was playing when he shot himself. She explains to Mitch why she turned to promiscuity: "After the death of Allan—intimacies with strangers was all I seemed able to fill my empty heart with."

Then the street vendor comes to the door, and Blanche turns her away, slamming the door in fear after her. The "flowers for the dead" interruption seems eerily timely given Blanche's recent words; it also stirs in her the memories of her elderly relations she had to watch over as they died—trying to deny the reality of death the whole time. Blanche continues, "Death ... the opposite is desire. So do you wonder? How could you possibly wonder!" This reveals a side of Blanche Mitch and viewers haven't seen before, giving her backstory depth and sympathy. To escape the horror of "blood-stained pillow-slips" and "the long parade to the graveyard," Blanche turned to sexual liaisons with soldiers from the nearby training camp. After her intimacies with them, "the paddy-wagon would gather them up like daisies." After the word daisies, and throughout this section, the Mexican woman repeats her cry, "Flowers for the dead." 

The Mexican woman's appearance creates the objective correlative of death that punctuates the scene. Without a tangible representation of the presence of death that drove Blanche to drown her pain in promiscuity, this scene would be much less powerful. The woman and her chant help viewers experience more deeply the emotions that Blanche describes and displays during this scene.

In my opinion, Blanche sees the street vendor you ask about in your question as a prophet.  As such, she is a very important minor character in A Streetcar Named Desire. Her appearance (and significance) can only be found at the very end of scene 9 in A Streetcar Named Desire.

The beginning of scene 9 in A Streetcar Named Desire involves another semi-tragedy for Blanche. Mitch breaks up with Blanche. At the end of the scene, and as a result of her pain, she runs out of the apartment to the street as a result. This is when Blanche sees the street vendor. The description of the street vendor is interesting. In addition to not being seen previously in A Streetcar Named Desire, the street vendor is female, blind, and definitely of Hispanic origin (very likely from Mexico). She is selling flowers as she declares her famous line:

Flores, flores para los muertos. ... Flores, flores para los muertos.

Translated into English, this means "Flowers, flowers for the dead. ... Flowers, flowers for the dead." Blanche is flabbergasted. The woman, as innocuous as she may seem, seems to be pronouncing Blanche's death. This is why I, as a reader, see her as a prophet. It is at this very moment that Blanche realizes that she has nothing left. She has lost it all. It isn't long before she is labeled "insane" and forced into an asylum.

The street vendor, who appears at the end of scene nine (in Tennessee Williams' play A Streetcar Named Desire), is a blind Mexican woman who is selling flowers. Blanche has just rushed out of the flat, to the street, after being "dumped" by Mitch.

Once on the street, Blanche hears, and sees, a woman peddling flowers to passersby. The woman is calling out to those around her saying "Flores, flores para los muertos." (Translated, this means flowers, flowers for the dead.) Blanche sees the woman as an announcement of her coming demise.

At this point in the play, everything has fallen apart for Blanche. It is not until Blanche sees the woman selling flowers that she truly recognizes the fact that she has lost everything. Therefore, the Mexican woman selling flowers signifies Blanche's complete downfall and, ultimately, her death.