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