Well, it's full of conventionally dark symbols: the crow, hemlock, and cold winter season. And the diction echoes this with words like shook, dust, and rued. Other word choices could have told a very similar story in a more cheery mood, if desired.
It's also a vignette devoid of human contact, which is common to many of Frost's more introspective poems. The narrator is the main character in an empty or lonely scene, except for a brief interaction with an animal.
Even the choice to say "change of mood" delays clarity until the words "save" and "rued" provide disambiguation. Yet it is also quite short, almost an epigram.
So while the final message is one of positive valence, it is rooted in dark humor, almost a wink emerging from a scowl. I think this is an example where Frost is revisiting some of the ground shared with one of his early influences, Edgar Allen Poe.