Coincidentally, it's exactly how I just distinguished them in English just now.
You can read ChatGPT's replies below (I manually collated the replies, leaving my prompts out):
In Japanese, mackerel is written as 鯖 (さば).
Blue fish" in Japanese is 青い魚 (あおいう お).
The middle character in "青い魚 (あおいうお)" is い, which is a form of the auxiliary verb "to be". In this case, it indicates that the noun "fish" (魚) is in a state of being "blue" (青い).
By using the auxiliary verb "to be", the phrase "blue fish" (青い魚) can refer to any fish that is blue, as opposed to a specific type of fish like mackerel (鯖). This helps to avoid confusion and allows for a clear distinction between different types of fish.