Right so to answer this, ideally we'd need to go into some background on basic colour theory and how not all cultures divide the colour wheel into the same number of slices. But that'll take a while, so I'm going to skip it for now and just say that in traditional Chinese culture, there were 5 primary colours. They were Red, Green, Yellow, Black and White. Actually this is also a little imprecise and we might actually want to call Red "Warm" instead, and "Green" cool instead, but we'll just use these English terms instead for the sake of simplicity. Just remember that the "red" 朱 used in the Chinese context won't necessarily match up with everything you think is "red" in English. So, anyway, 5 colours.

There were also five elements. Wood, water, fire, metal, earth. Wood matches up with what we're calling Green, which was written 青 and also included what we'd call most shades of "blue" in English. Red is the one I already gave above, and was assigned to Fire. This makes sense on both of these, as trees are green and fire is red, again for simplicity's sake.

Earth was yellow, also fair, considering the loess plains in Central China, metal was white, and again this is consistent. Know that what you might call silver or chrome is called White in a lot of cultures. That leaves water, black 玄.

Now, this word, today pronounced xuán in Mandarin, has a number of other meanings. One of those is "deep", as in both thought and as in the ocean. It shows up in the last verse of the first chapter of the Dao De Jing with the meaning of "mysterious" or "unknowable". So this too really isn't much of a stretch for water if you're considering water better represented by the seas than what's in your cup you're drinking from.

Going back to the basic colour terms, "blue" and also the modern word for "green" are both newer terms in Chinese languages. At the time that the system of five elements was developed and in use, and certainly throughout the classical period, there was no sociological division between what we're calling blue today in English and what we're calling green. In fact, things you'd call black now could also be referred to by this term 青 which is often translated "green" in the cosmological system we're talking about here.

There wasn't a basic colour term for blue as distinct from the basic colour term for green. And if there were, the balance of the fives wouldn't have worked out anyway. And regardless, you don't really get a basic colour term for "blue" until you already have one for black, white, red, green and yellow.

Hope that all makes sense.

Let me know if you want any of this expanded upon. I actually do a fair bit of work on this topic, including fieldwork with undocumented language groups, so I'll be happy to chat about it if you end up with specific questions.