Blueberries have blue skin, but the inside of the fruit is reddish. Why is this?

Research published in Science Advances shows that the blue hue of blueberries does not come from pigmentation but from a layer of wax surrounding the fruit.

This nature-made optical illusion occurs because the skin is composed of miniature structures that scatter blue and UV light, appearing as a blue hue to the human eye.

The study shows that this anomaly usually occurs in 'dark pigmented fruits with wax blooms,' such as blueberries, plums, and juniper cones.

To demonstrate the 'epicuticular wax bloom characteristics,' researchers removed the wax and re-crystallized it. The color appeared blue in human vision, but when looked under the microscope after removing the wax, rinsing it with chloroform, and immersing it in oil, the fruit's flesh appeared red. 

The ultra-thin colorant is around "two microns thick," visibly blue, and a great UV reflector.

Scientists were able to reproduce the color on a new blue coating by harvesting the fruit's wax. The findings could lead to more sustainable, biocompatible, and even edible UV and blue-reflective paint and colorant methods. 

Most plants are coated in a thin layer of wax for various reasons, many of which scientists still do not fully understand.

Blue fruits, however, are rare. They are so rare that, according to the study, blue fruits constitute such a small group that they have been classified as outliers in global-scale reports and studies.