It brings to mind a children's book about the quest for a blue banana.

Other than that... cake icing, passionfruit flavoured things, raspberry flavoured sweets (though I've never understood that logic), a particularly nice ice-cream flavour called Blue Moon. Almost all very sweet, and thus things I can hardly eat now. Hm... blue kale, some rare varieties of cabbage... rue and some varieties of lavender, if flavour ingredients count as food.