“Down in the mouth” vs “Blue”

Question

I wonder what would call someone who seems to be unhappy and a bit sad in informal English? In the state in my question, the person doesn't show tendency in talking to the surrounding people and keeps silent and is quite visible from their appearance (comparing their normal status) that they feel sad or at least not in the mood!

Example:
A) What's wrong with you Paul?! Are you OK?
B) I'm OK mom! (said Paul unhappily.)
A) But I don't think so son! You look a bit..................

I know two similar terms which based on published definitions can be used properly and interchangeably in this sense. I would appreciate it if you could let me know what do you think about them and how do they sound to you here and in general.

a. down in the mouth.
b. blue.

Answer 1

Both of these are idiomatic expressions that sound a little old-fashioned. I can't recall the last time I heard any contemporary say or write "blue" or "down in the mouth", outside of something like song lyrics.

So using either of these sounds a little forced, like you're trying to find a good context to use an expression you learned, instead of letting it flow naturally. Instead I would say simply "sad", or a synonym like "troubled" or "upset", or possibly the idiom "down".

But that's just with me and what I read. Other people might be fine with them.

Answer 2

Probably this example should use "down in the mouth" since that is more likely to be something that is visible. It simply refers to a frowning expression. The corners of your mouth turn downward when you frown so "down in the mouth." The term "feeling blue" is an emotional state. One does not actually look blue when one feels blue.

Of course, if Paul is actually blue maybe he is very ill.