Blue is not only the heroine's name.
Blue represents:
Feeling Blue
sea
sky
peace after growing up
The story is actually about:
How a girl transforms from a "substitute" to a "chosen one".
"Lan has grown from a girl who was willing to be a substitute to a person who can live a good life even if she loses Leo."
Concept: How a girl changes from a "substitute" to a "chosen one".