This short poem hides many symbolic references.
Snow is frequently thought of as being pure and white, a cleansing and peaceful part of nature not usually associated with the dirty content of "dust."
The crow, with its black coloring and loud, discordant call, may be considered bad luck or a bringer of ill fortune.
Some types of hemlock are poisonous when ingested by humans, which would make the crow in the hemlock tree appear all the more ominous.
And yet, the unexpected "dust of snow" has lifted the spirits of the poem's narrator. The speaker's heart experiences "a change of mood" as a result of the snowy surprise, making brighter "a day I had rued" - a day in which he was feeling sorrow or regret became brighter and more positive in outlook after the cleansing of the snow.