That’s not how word classes are defined in linguistics. If words inflect and they belong to the same class, they inflect for the same categories (e.g., tense or number), but not all members of the class have to have the same inflectional forms. This definition of word classes applies crosslinguistically.
In order to belong to the same class, words have to behave in the same way or serve the same function syntactically, which both types of adjectives do in Japanese. They are a word that modifies a noun or noun phrase. They also behave identically in derivation processes.
Exceptions, by the way, do not exclude a certain word from belonging to the word class as long as it meets all other requirements and, importantly, cannot fall into another word class because it doesn’t share its properties. To use English as the most accessible example for everyone, there are adjectives that cannot be used predicatively for semantic reasons (e.g., former – we can say the former president but not the president is former), but they are still adjectives.
Apart from that, if adjectives like 静か were nouns, you wouldn’t be able to nominalize them using the nominalization suffix –さ as you can only nominalize something that isn’t already a noun, which is why 男 and –さ don’t go together.
In your example, blue-collar is an adjective that modifies the noun job. It cannot be anything else because English doesn’t have any other word class that can modify a noun in that syntactic position. As @Belthazar pointed out, English famously employs so-called zero derivation or null derivation to form new words, i.e. a word can change its class without any affixation (unlike, for instance, 静か and 静かさ), which is why the phrase blue collar can be used as an adjective.
You can easily test this by looking for environments in which only adjectives occur, e.g. a noun modifier in the superlative, and you will find sentences such as: Rizzo had been a police officer, arguably one of the most blue-collar occupations in the city. 
This is an example of a compound noun where two nouns combine to form a compound, which in turn is also a noun. I’m not quite sure what you were trying to illustrate with this example? In either case, all Germanic languages use the same method of compounding (Winterkleidung, winterkleren, vinterkläder etc.) and, coincidentally, Japanese does too, cf. 冬服.