Why are they called the Blue Mountains

One of the main factors of the eucalyptus tree is the oil found in the leaves, which are used for a number of benefits for humans. However, the scorching sun of Sydney affects these trees, slowly heating them up until the oil seeps out in a mist. This oil fume mist covered the area, creating a beautiful crisp bush smell within the region. Although the mist cannot be seen up close within the mountaintops, the mist can be seen reflected by the light at a distance, making the region looking like it is engulfed in a blue haze. When looking at the mountains from Sydney in particular, the oil fog covers the entire range, resulting in the mountains to look blue from afar.

https://bluemountainstoursydney.com.au/blog/why-are-blue-mountains-blue/