Ask a grown-up: why are blueberries blue?
Blueberries aren't actually blue, but deep purple, which is the colour of anthocyanin, a pigment that is especially rich in blueberries.
Humans have evolved to be attracted to, and to want to eat, coloured foods. For example, our ancestors would have known to eat berries that had a rich colour, because that meant they were sweet and ripe. Animals would also have been attracted to the vivid colour of ripe berries, and would have eaten the berries, resulting in the seeds being spread around afterwards when they went to the toilet. This effectively ensures that there are more berries available the following year, so it's a clever way for a plant species to survive.
Blueberries are known to be high in antioxidants, which are good for the human body; the anthocyanin is thought to be useful for combating inflammation. A good rule to follow is, the darker the berry, the more anthocyanins are present.
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