Hello! I guess that this very short phrase is some sort of order, you are telling someone to blame Nick. Therefore, it is an imperative form whose opposite would be "don't blame Nick". Other examples of phrases with imperatives are: "make your bed" "be good" "tell him to shut up" "don't be stupid" "don't laugh at me" "go outside now" However, these phrases are very direct and English is a language that is full of politeness, so I'd suggest avoiding these phrases unless you are with someone you have a close relationship with or you are very angry about something. I hope to have helped you!

As a complete sentence, it's an imperative (a command), with the subject, "you", understood tacitly. As part of a longer sentence, it may be called the predicate, or (if there's more context) perhaps a bare infinitive phrase. Those are your search terms: imperative, predicate, bare infinitive phrase