This usually means that the character is depressed, This has happened with my families on several occasions. An example - in one of my families, the dad passed away. I started the next generation by bringing home one of the sons. His status code was "a bit blue". I gave him fruit, vitamins, and candy. His status immediately changed to "happy" after receiving a marriage proposal and accepting it
Sometimes a child is "a bit blue" when he or she is the only child and a brother or sister is born. I give the child fruit or candy and send him (or her) outside to play. This usually works for me!