Repetition can be used for effect.

The repetition in the song is: “The blacker the berry, The sweeter the juice. Kendrick Lamar is trying to say “The blacker you are, the better you are”

perhaps he is trying to say black people stand out.

Kendrick Lamar is trying to say instead of hiding your blackness, embrace it.

The repetition helps to emphasise the key point “being black is positive”

The song criticises the reel indirectly  by saying that “You hate me don’t you?” I think this means that because he is black that the white Americans don’t like him or his community. But he makes good points by saying “I know you hate me just as much as you hate yourself.”