Usually when someone says they are ‘blue’, they are generally feeling down/sad. Here, Frank is ‘blue’ over Forrest (his crush/love he talks about in the song). Also, blues is a type of music, generally is sad/sappy, consisting of wistful lyrics about a broken heart or lost love. He’s singin' the Forrest blues.
This can also be a play on words. he says forrest green “for his blues” meaning that when he is sad he smokes to forget about things.
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