Blueberry is the name of your latest musical opponent whom you have to defeat right now in the FNF Moonlight Berry mod, where you have a full tracklist comprised of the following tracks:
Help BF take on and defeat Blueberry of FNF!
Whether you're playing in the story mode or free play mode, winning is achieved in the same way, by reaching the end of the songs, and to do it you need to hit your notes according to the charts until that conclusion.
So, when you see arrow symbols match above BF, make sure to press the identical arrow keys at the same time. Don't miss hitting them too many times in a row, or you lose and have to start again from scratch. Enjoy, good luck!