Fwiw, I hold a degree in Horticulture. A good source is David Lee, author of Nature's Palette: The Science of Plant Color and a retired professor in the Department of Biological Sciences at Florida International University in Miami.

Maybe there's some kind of definition that make this make sense? On the face of it it seems very weird to claim that "this is actually a red pigment, it just looks blue". Why not say that it's a blue pigment which looks red in certain pH conditions?

I think our disagreement is over blue pigment vs blue appearance. Lots of things in nature use structural coloration https://en.wikipedia.org/wiki/Structural_coloration

Microstructure interference can create blue, but perhaps not the flower pigments themselves

The article seems nonsensical in the same way, though.

> For plants, blue is achieved by mixing naturally occurring pigments, very much as an artist would mix colours. The most commonly used are the red pigments, called anthocyanins, and whose appearance can be changed by varying acidity.

It's just saying the same thing, "a red pigment appearing blue", without any explanation. (Also, as artists know, blue and red are primary colors, you can't get blue by mixing red paints.)