Blue is the colour of the sky. That's what Willow's picture books said, and Xander's comics – graphic novels, as he later insisted that they be called – agreed, cheerfully. California's weather obliged, and for a long time, or a short time that seemed like a long time, blue was the colour of idyllic childhood; blistering midday sun through the sky on the green in front of the high school, where she and Xander lay in wait to ambush less-than-happy students. It melted the white, hazy desert-like heat out on Kingman's Bluff, when Xander sat with a really-just-borrowed cowboy hat from Partytown, a miniature Indiana Jones. Yes; for a while, those blue-sky days formed Willow's entire view of life.
Her first day of school, dressed in the neatest of starchy skirts to make a good impression on the kindergarten teachers, was a blue-sky day. Maybe that tilted her view of education in general to the better, or maybe it tainted her appreciation of blue-sky days for the worse. Whatever happened that day, as Willow got older, the sky continued to be disgustingly blue, but something about the rest of the world became greyer. Maybe, as a world-weary ten year old, she had left the days when a blue sky was something special behind. In California, you saw a lot of blue, after all. Days were spent at home instead of on the green, in her room instead of on the Bluff. Willow became more intimately acquainted with the Blue Screen of Death than with the sky.
"The sunlit sky appears blue because air scatters short-wavelength light more than longer wavelengths. Since blue light is at the short wavelength end of the visible spectrum, it is more strongly scattered in the atmosphere than long wavelength red light. As a result, the human eye perceives blue when looking toward the sky."
A few years passed, and blue meant Buffy's high fashion LA wardrobe, and really nasty gooey demon blood that was impossible to get out of any clothes, and saving the world on Tuesday afternoons. It meant stargazing in graveyards. It meant that Willow was growing up.
With Oz, blue skies meant summer – meant long, lazy days where night fell late and short. It meant that those three days a month where Oz was something else were cut short.
Blue was the music Oz listened to, and, by extension, the music that Willow listened to as well. Blues in a garage on a summer afternoon or a winter morning – not the evening, when Willow was busy or Oz was… elsewhere. Blues with Oz on guitar and Devon singing, messing around, making Willow giggle a bit, feel grown up, feel almost cool.
The day Oz left was a grey day, but Willow certainly felt blue.
"The phrase might also be linked to a custom among old deepwater sailing ships. If the ship lost the captain or any of the officers during its voyage, she would fly blue flags and have a blue band painted along her entire hull when returning to home port."
Blue-sky days were a long way away now, in a time of innocence. That's what it is: blue means innocence. Innocent blue like Tara's eyes, as they look at her now, full of trust, full of love, forget-me-not blue that's forgotten what Willow said and did the night before.
Willow smiles and lets herself sink into that innocence, lets herself pretend that it's a real blue-sky day again, swinging Tara's hand in her own. She's aware that Tara asked her a question, joking, laughing innocently, so she answers.
"Well, who wants to be cooped up on a day like this? The sun is shining, there's songs going on..."