I know that later in the article the language changes to “scarce” instead of “does not exist” but blue, though uncommon, is certainly a thing.
Most prominently in my garden, the Thai Butterfly Pea Flower [0] is blue, and the tea from it is blue ( though it goes purple when honey or lemon are added )
Honorable mentions include Bachelor’s Button, Borage and verbena, though I’m sure there are likely arguments about color in some of those.