Natural Blue Food Colour
Despite being a primary colour in the visible spectrum, there are very few naturally-occurring blue foods. However in part due to it's rarity, there are several food applications where a blue colour is required.
This could be for a blue beverage, blue sugar confectionery, blue coatings for sweets or nuts, other blue sweets. The fact it is unusual makes the colour striking and appealing, despite the colour not being associated with any nutritional benefits.
Sources of blue food colouring are obviously limited since so few foods are naturally found with blue colour however the following have been used successfully: Spirulina Blue, Gardenia Blue, Blue Sweetcorn, Indigo Blue (from the blue flowers of a plant Indigofera Tinctoria). Natural violet food colour is also possible using with appropriate blends of colouring materials.
Natural blue colours have challenging stability and application limitations so experience and expertise in their use is required.