Что означает выражение «not so blue» (см.)?

Выражение "not so blue" используется в песне "Liar liar" группы The Castaways. Вот текст песни (см. последнюю строчку второго куплета) :

Chorus:

Liar, liar, pants on fire

Your nose is longer than a telephone wire

Verse 1:

Ask me, baby, why I'm sad

You been out all night, know you been bad

Don't tell me different, know it's a lie

Come kill me, honey, see how I cry

Verse 2:

Why must you hurt me, do what you do

Listen here, girl, can't you see I love you

Make a little effort, try to be true

I'll be happy, not so blue

[repeat chorus]

Verse 3:

If you keep on tellin' me those lies

Still goin' out with other guys

There'll come a day I'll be gone

Take my advice, won't be long

Verse 4:

When that day comes, won't be mad

Be free of you, but I'll still be sad

In spite of your cheatin', still love you so

I'll be unhappy if I let you go

[repeat chorus]