Most languages provide ways to shuttle morphemes between word categories. Sometimes the transformation adopts a phonetic exponent ("small-ness"), but sometimes not ("three smalls").
To argue that there aren't actually lexical adjectives, on the other hand, requires quite a bit more information. Specifically, you want to analyze the distributional criteria of semantic "things" and "properties". I suspect you'll find some restrictions, though it's possible you won't.
In any case, if you have phrases like "the blue book", the fact that you might also have a phrase "the blues" doesn't actually suggest that "blue" was covertly a noun all along. That's where I'm losing you I think. If this language doesn't have adjectives, and consequently if "blue" in "the blue book" isn't an adjective, what is it?