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.
To help improve the meaning of these lyrics, visit "Forrest Gump" by Frank Ocean Lyrics and leave a comment on the lyrics box