Personally, I don't think 'Blue Valentine' is a "cute" song but rather emotional, heartbreaking and vocal heavy. This song is their most vocal heavy title track, which is one reason why a lot of people love it so much. Also, Sullyoon is the one doing most of those ad-libs during the final chorus of the song, Kyujin only does the "Always be my blue valentine" ad-lib at the end. Just in case you didn't know already.