In the final scenes of the BoJack Horseman ending, the show plays out with Catherine Feeny’s song “Mr. Blue”, which becomes a poignant capstone on his relationship with Diane. The song speaks of a tumultuous yet loving relationship, when they care for one another, but it’s ultimately unhealthy. The lyrics, such as “I have to go now,” reference Diane’s move to Texas, and leaving BoJack and "Hollywoo" behind. It backs up just how important her decision is for her own health and happiness.
The lyrics of "Mr. Blue" also refer to BoJack’s own future, and how though audiences have to leave him behind, it's clear he'll actually be alright. "Mr. Blue, Don't hold your head so low that you can't see the sky / Mr. Blue, It ain't so long since you were flying high.”