Saw a blue food on Iron Chef Gauntlet (OK, I am addicted) and one of the (failed) contestants included some pea flower in it. This turns deep blue when acidity hits it (it is light blue to start with.
The butterfly pea flower, also known as “Asian Pigeonwing,” is employed prominently in Malaysia to naturally color rice and cakes, and in Thailand, where it’s served as a syrupy welcome drink. It’s even sometimes consumed with citrus juice, in which case the high acidity turns the substance to a pink-purple.
Other than blue Curacao and berries I can't think of too many blue foods. Some even suggest putting a blue light bulb in the fridge or freezer to help those on diets. Anyone tried the pea flower trick?
More on why blue food isn't popular:
https://www.colormatters.com/color-and-the-body/color-and-appetite-matters
https://www.thekitchn.com/how-color-affects-your-percept-96524
Starbucks is now doing blue coffee drinks (Asia only) using this effect.