adjectives - What is the difference between あお and あおい?

I know あおい means "blue (adj.)" and あお means "the color blue". What does "blue (adj.)" mean? Like "I am feeling blue?"

Note I have just started with Japanese. I am on first column of Hiragana. I did do google search but I wasn't able to understand the answers that come up.

Answer

Technically, 青｛あお｝ is a noun. You would use it as you would use other descriptive nouns (i.e. noun is [descriptive noun]). For example:

ボールは青｛あお｝だ。

The ball is blue.

空｛そら｝は青｛あお｝だ。
The sky is blue.

Obviously, construction can get more complex, but the general idea will stay the same. You'll get the hang of it when you start to deal more with descriptive nouns.

青い｛あおい｝ is an adjective describing something as blue. For example:

青{あお}いボール
blue ball

青｛あお｝い空｛そら｝

blue sky

The idiom I'm feeling blue will never have a literal translation in Japanese, since 青｛あお｝ and 青い｛あおい｝ are only used in relation to color. Colors typically cannot be applied to feelings like the English counterpart is in this idiom.