how does williams use the contrast between Blanche and Stanley to represent different aspects of american society

Blanche represents old world refinement (false as we come to see), and Stanley represents those of the struggling working class; he's rough, brutish, and doesn't hide his feelings behind a facade of meaningless words.

Ultimately, they're very alike, and we find that the different aspects of American society aren't so very different. People are people, and A Streetcar Named Desire portrays people, although it does so with a large degree of exaggeration.