The poem is about looking at two ponies (nature) and feeling transcendent as a result.Many other things are suggested by the language the author uses. For example, the feminine nature.
Feminine nature is a subject in the poem as you can see from the narrator's stating that one pony is a female: "she has walked over to me" … "her mane" … "her long ear / That is delicate as the skin over a girl’s wrist."
Another subject the poem deals with is death. The poet is telling us that when people die they don't just vanish. The image of stepping out of the body shows us that he feels at one with the pony and nature.He is not just a human being but part of nature. He can transcend his human (body) nature.
Because of the poet's rapturous encounter with nature which seeing the ponies represents, the reader of "The Blessing" can understand that the poet believes in transcendence. In other words, that he is more than a mere mortal. This awareness he has makes him profoundly scared, as if he would "break". He is also feels he has become pure, like a "blossom".