The sky doesn't contain any blue pigment, either. And a blue-eyed person has no blue in their irises.
You're referring to pigment; there is no blue plant pigment, true, but that seems a little pedantic, even if it's interesting. Plenty of flowers reflect blue light, which is what people mean when they say an object is "blue".
What? I have a bunch of hydrangeas in my yard right now that range from light to dark blue. They are definitely not violet or any other close blue color. They are as blue as blue can be. And they haven’t been artificially tampered with. We planted them 15 years ago and have done nothing for them since. Not even fertilizer.
Those plants that do appear blue are in fact often using a red pigment known as anthocyanin. Through pH shifts and a mixing of pigments, combined with the reflection of natural light, the plants are able to generate the appearance of a naturally occurring, blue color. That's the reason why plants such as bluebells, hydrangeas and morning glories appear various shades of blue, when in fact, as Lee explains, "There is no true blue pigment in plants."
> through [...] mixing of pigments, combined with the reflection of natural light, the plants are able to generate [...] blue color
We have a way to describe objects which “generate a blue color when light shines on them”. We say they are “blue” objects.
Likewise, bird feathers or butterfly wings which absorb/reflect light based on their small-scale structure and appear blue are still called “blue”, even though they might be colorless when pulverized.
What does it matter whether they have blue pigment or not? If nearly every person looks at it with their own eyes and say, "That is definitely blue" then it is blue. I don't care why or how it is blue. Anything else is just pedantry.
Some of the "blues" generated in nature rely on specific physical structure or other tricks to appear blue. This means they reflect predominantly blue light, but may not retain that "blueness" when ground/processed into a pigment. That's one reason there are few natural organic blue pigments, even the ancient Egyptians relied on chemical solutions for "Egyptian Blue".
Interestingly, the one dye listed in the link above that came from a plant is indigo, which is extracted from the green leaves of the plant https://en.wikipedia.org/wiki/Indigofera_tinctoria . There's no animal or plant I know of that both appears blue and could be used to create even a poor blue pigment, though I'd definitely be interested in any exceptions I haven't heard of.
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?
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.)
You're referring to pigment; there is no blue plant pigment, true, but that seems a little pedantic, even if it's interesting. Plenty of flowers reflect blue light, which is what people mean when they say an object is "blue".