Because blue is a rare pigment in nature and animals don’t find it appetising (so very few plants produce blue fruit).
There’s some debate about whether any natural blue foods actually exist.
We think that’s stupid because blueberries look blue – and looking blue is the only requirement of being blue.
So they’re blue.
Moving on, blue foods are certainly rare – and many people have puzzled over the reasons for this.
It seems that the compound in plants that usually produces blue colouring – anthocyanins – also produces red. And even a little red mixed with blue makes purple.
It is also argued that red stands out more against the green leaves of plants, and that animals respond better to red than blue when looking for food.
“We don’t have an automatic appetite response to blue,” says Dr Farah Hosseinian of Carleton University.
So fruit-bearing plants – which rely on animals to eat the fruit and deposit the seeds (along with a helping of manure) – have not evolved to produce blue food.