Images of light and darkness are everywhere in poetry. In fact, if you went to shop at the poetic devices store, the most crowded aisle would probably be the one where all the light and dark images are stocked. But, as we've noted elsewhere, Louise Glück is kind of a minimalist, and she doesn't appear to have hoarded many images of light/darkness, at least for this poem. Still, she takes full advantage of their potential as poetic symbols. Though the word "dark" appears only once, that's all it takes. The darkness of burial and death overshadows much of the poem.
Title: As we've seen, there's some wordplay in the title, as the word "iris" can refer to a Greek goddess as well as a flower. But there's yet another meaning for iris: the colored part of the eye. That means that this image is connected to light, because we perceive light through our eyes. And sometimes the iris of the eye is blue.
Lines 6-7: Here's some sunlight, and it's associated with signs of life (at least there's a pine tree growing nearby). Still, the light is "weak" and intermittent—not too cheery.
Line 10: As promised, here's your one explicit image of darkness, and it's connected to a burial. Even though the speaker doesn't mention darkness again, we don't really get out from under that shadow of death until the last stanza of the poem.
Lines 21–23: The visual image of the fountain seems to banish all the darkness that preceded it: "azure seawater" drenches the scene in beautiful blue light.