Can someone explain the symbols of the Elysian Fields, dark and light, bathing (water)?

The light symbolizes Blanche's past and reality. You'll notice she stays away from light, covers the lights in Blanche's apartment, and will only see Mitch in the evening. This is because the light reflects the reality of who she has become, and she doesn't want to live in the reality of an older woman, destined to end her life alone.

The frequent bathing symbolizes Blanche's desire to cleanse herself of the past.

Elysian Fields is the name of the place to which Blanche has relocated. The Elysian Fields are a mythological place of happiness, the place where the blessed go after death. As you can see, the author has provided us with this purposeful bit of irony because Blanche will never be happy there.