I copy this explanation from Blue Mountains' website, it will explain it well:
"The Blue Mountains is densely populated by oil bearing Eucalyptus trees. The atmosphere is filled with finely dispersed droplets of oil, which, in combination with dust particles and water vapour, scatter short-wave length rays of light which are predominantly blue in colour."
In my second photo, part of the blue was enhanced in Photoshop (as in China, people usually say "源于生活，高于生活“ ）
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