Romeo and juliet why is "pernicious blessing" and oxymoron

An oxymoron is two words with opposite meanings, that mean something together, like "jumbo shrimp"
Pernicious means "having a harmful effect, especially in a subtle way" and a blessing is a good thing. 
I'm not sure what context those words are used in, or what they are used to describe, but the phrase is by definition and oxymoron.

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