Both dramas deal with serious themes, difficult relationships and hard life situations.

Relationships in Left Right are quite dysfunctional and they are more about family.
Relationships in More Than Blue are mostly romantic but still have to deal with a lot of hardship.

Both dramas will completely destroy you and leave you crying with each episode.

They are similar in themes but More Than Blue is more gentle/poetic/almost lyrical while Left Right is very harsh/realistic/dark.