It's all about making the traveling experience less stressful and blue is said to induce a feeling of calm.

这一切都是为了让飞行体验不那么紧张，据说蓝色可以让人产生一种平静的感觉。

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youdao

He'd been feeling blue all week.

他整个星期都郁郁不乐。

《牛津词典》

Your doctor will ask about whether you've been feeling down or blue, whether you've lost interest in things you usually enjoy, whether you feel guilty or especially down on yourself, whether your sleep and appetite are disturbed, whether you feel exhausted most of the time, whether you are having trouble concentrating and whether things have gotten bad enough that you've thought some about ending your life.

CNN: Stress has taken over: How can I turn my mind off?

You know that feeling on a clear blue sky day.

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And they will not be able to receive surprise presents, like special cookies or favorite movies, which are often brought to the space station on supply shuttles when someone starts feeling homesick or maybe a little blue.

CNN: Mars on the brain? Red Planet pioneers to face cosmic mind trip