To feel blue means to feel sad.
I don't want to go out. Since my boyfriend left me I've been feeling blue.
Don't feel blue. Life is short, have some fun!
When you say something will happen until you're blue in the face you mean it will be happening for a long time.
You can talk until you're blue in the face. You are not my boss and I'm not going to follow your orders!
Despite their popularity in the aquarium trade, mandarin fish are considered difficult to keep, as their feeding habits are very specific. They are also sometimes called psychedelic fish due to their unusual colouring.