You are talking to a blind man who asks you to describe the colour of the sky. Write down what you would tell this man without once using the word "blue" in your explanation.
The closest feeling is a refreshing sense of coolness. Imagine chewing a mint and the feeling of freshness in your mouth. Imagine yourself being enveloped by this coolness, that's not cold in terms of temperature, but refreshing in terms of vitality. It makes you take a deep breath when you see it. A wide open expanse as far as the eye can see, only further. That feeling of lightness and spaciousness - all at once you feel like you're floating like a balloon up into the sky, and expanding like a balloon, only infinitely in all directions of time and space. And after all that freshness and lightness, comes the deep calm and peace. There's nothing left to desire. No more things to achieve, no further steps to take. You feel like one tiny thing in an endless medium embracing you, yet that same medium is also you.
I found this awesome resource for writing prompts here. Since I'm still lost about what to write, I guess trying out some of these activities (like this one called "Blue") will be fun!