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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.
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.
ボールは青いです and traffic lights
Jun 26, 2018 at 18:00
As you mentioned before, あおい refers to blue as an adjective, and あお as a noun "the color blue." Addressing your question directly, あおい as an adjective is referring to the property of an object that is color.
If you think of an adjective (in this case, an い-adjective that is あおい) as a word that tells you about a property of an object, you might be able to tell that there is some usage overlap that can give you a similar meaning between the adjective and the noun counterpart for this word. However, a clear distinction needs to be made that is of the nature:
X(noun) は Y(noun) です
X is Y. (X is equivalent to Y.)X(noun) は Y(adj.) です
X has a property which is Y.
It is possible to state the following with both words:
This ball is blue.
このボールはあおです。
このボールはあおいです。
...However, the meaning is slightly different between the two:
このボールはあおいです。
This ball is blue. (lit. is of a blue nature, has a blue "color" property, etc.)
...versus:
このボールはあおです。
This ball is blue. (lit. this ball is LITERALLY "the color blue").
As for using the color あお as denoting the "blue" feeling, this is a concept that is not native to Japanese. However, you could use ブルー "blue" to say this. This is not something that everyone will understand universally, but is present in at least some form of popular media (the song 仲良し by Spitz comes to mind for this). The lyric that contains this usage is:
いつも仲良｛なかよ｝しでいいよねって言｛い｝われて、でもどこかブルーになってた、あれは恋｛こい｝だった。