As symbols, darkness and light should be old friends to you by now. You've probably encountered them in any number of poems. Often, light has positive associations—life, hope, love—while the absence of light symbolizes all kinds of bad stuff, such as fear, despair, and death. Like almost every other poet in the history of the world, James Wright uses darkness and light as symbols, but not in the way you might expect. Instead of assigning clearly positive or negative values to light and darkness, he blends images of darkness and light to convey an "in-between" state of mind, setting the stage for a mysterious shift in awareness.
Line 2: Long ago, before the word "twilight" was forever linked to vampires, and even before the spooky television series The Twilight Zone (ask your parents), "twilight" usually just referred to the period of half-light after sunset before night. Though often considered beautiful, twilight is also associated with uneasiness, as the dim light can play tricks on your eyes. When "Twilight," in personified form, "bounds softly" into line 2 of "A Blessing," its unexpected presence establishes a tone of mystery and even magic that suffuses the rest of the poem.
Line 4: Eyes can "darken" for various reasons, not all of them pleasant; "darken with anger" is one common turn of phrase. So it's a little surprising to learn that the eyes of the ponies are darkening with "kindness." See what we mean about Wright's unconventional use of light and dark symbolism?
Line 8: The phrase "grazing all day" conjures up the image of long, sun-drenched hours preceding twilight. Such a pastoral scene often connotes quiet contentment, and the following line does include the word "happiness." But line 8 ends abruptly with the word "alone," foreshadowing the "loneliness" in line 12. So rather than having purely positive connotations, the light associated with "day" conveys a mixed mood.
Line 14: Ah ha, darkness again! But again, it doesn't have the negative symbolism we might expect. In fact, the prepositional phrase '"in darkness" literally contains the positive symbol of "spring" (see the "Spring" section), with all of its hopeful connotations of youth ("young"), growth, nourishment ("munching"), and hope.
Line 18: While twilight blends darkness and light, creating a kind of gray light, the ponies' "black and white" markings create an alternating pattern of darkness and light, not unlike the symbol of Taoism, an ancient Asian philosophy (see an example here). The black and white markings of the Tao symbol represent the balance between opposing forces in the universe as well as the universal energy that flows through all things. (If you're interested in these ideas, check out the discussion in the "Title" section.)
Line 20: When you first see the word "light" in line 20, you might expect more light vs. darkness symbolism, but then you realize that the word is being used as an adjective to modify "breeze," so it means "gentle." Still, poets love words with multiple meanings, and nobody is going to stop you from momentarily considering "light" as a visual as well as tactile image, further reinforcing the positive associations of the breeze which leads to the speaker's mystical vision.