The night whose sable breast relieves the stark. White stars is no less lovely being dark. Is this an example of imagery, allusion, ambiguity, or metaphor?

It seems to me that this piece would either be an example of imagery, or an example of a metaphor.

Explanation:

An allusion means to reference someone else's work. Ambiguity is like a play on words. It just seems to me that this would either be imagery or metaphor over the other options.