I think that the theme of this poem is that people can and should see themselves as part of nature. They should not lose touch with the world around them. We see this in how the narrator and his friend think it's so important to get off the highway and go be with the horses. And we see it at the end where he says that he could (if he were blessed) break into blossom.
An important simile that supports this is that of the horse's ear and a girl's wrist. It emphasizes how similar people and animals are and how thin and delicate is the difference between the natural world and people.
I think the theme could be given away in the poem title 'The Blessing.' It could be read as being about the privilege bestowed on us sometimes by Nature and how we can, and should, not only appreciate it, but give thanks for it.
Perhaps the guys have had a long, arduous, dry dusty road trip. Perhaps they are glad to get out of the acrid, hostile, airless environment and have taken the first opportunity to 'get some air.' They find it is sweet, pure country air that has the fragrance of of twilight grass. Or perhaps it was the horses and the sight of their natural beauty that makes them stop in the first place. Either way, they are grateful for the precious moment to commune with Nature, both to the horses, to nature itself and maybe to God.
'Wet swans' is another simile and makes us think of serenity, the cooling balm of evening dew and silence - the peace and quiet of Nature so different from the frenetic buzz of the highway.
You may like to compare this poem to Robert Frost's 'Two Look At Two.' You can read about that here: