Twilight is that transitional time when day is transformed into night. In this case, twilight is also transformed, through personification, into a kind of magical entity, setting the stage for a mysterious encounter.
We step over the barbed wire into the pasture (7)
Purposeful change requires a willingness to enter unfamiliar territory, to move past old boundaries and meet new challenges. In "A Blessing," the speaker's willingness to step over the barbed wire clears the way for a transformational experience.
I would like to hold the slenderer one in my arms, (15)
Often we speak of the need to "embrace change." In the poem, the speaker is deeply moved by the mysterious beauty of the ponies in the pasture. The desire to enter more fully into this experience, to explore these new feelings, is expressed through the image of embracing the pony. But instead of following through on that intention, the speaker withdraws slightly, observing the desire rather than acting on it.
Suddenly I realize That if I stepped out of my body I would break Into blossom. (22-24)
In these final lines, the speaker's attempt to maintain objectivity—to observe an experience rather than fully participating in it—is almost overwhelmed by a full-blown image of spiritual transformation.