> “big dog” is an idiomatic phrase
Not a super common one though, and even if you were using the idiom you wouldn't write "black big dog" because it still sounds incorrect and makes it less clear you're using the idiom.
Not a super common one though, and even if you were using the idiom you wouldn't write "black big dog" because it still sounds incorrect and makes it less clear you're using the idiom.
Tangentially: it is, though; the two phrases are both valid, but have different meaning. “big dog” is an idiomatic phrase that, when used as such, functions as a single noun, which can be modified by “black” as a preceding adjective (if “black” is in the usual place between “big” and “dog”, the words have their individual meaning not that of the idiomatic phrase.)