This is super interesting, and as a fellow reader/writer of sci-fi, I think you have a fascinating concept here. For the record, I think you should try the Physical Sciences experts as well, but I'll have a go at it
The first thing we need to look at is "why is our flora green?" Well, because chlorophyll, sure, you might know that already. But what does chlorophyll do specifically that makes it green? The answer is in the absorption spectra: plants undergoing photosynthesis preferentially absorb red light, and don't use a lot of green light, so they reflect back green light into our eyes. (They use blue light because of its higher energy content per photon as well, but they rely more on red than blue.)
Why do they use red light? Most simply put, because our Sun emits a lot of it. As an additional notable factor, Earth's atmosphere also tends to filter that solar spectrum of light to selectively permit red light through more than green or blue light. Thus, plants have a lot of red light at their disposal, so we assume evolution has led them to develop red-using, green-reflecting traits.
In summary (and extensive oversimplification):
plants use the wavelength of light that is most readily available to them, and take the appearance of the wavelength of light they do not use. Availability is determined by the type of star the light comes from, as well as planetary atmosphere factors.
Now that we have that basic rule of thumb, let's work backwards from your planet. Your planet has blue and purple vegetation. That means they reflect blue and purple, so they must not like to use blue and purple. Now, thinking about the color wheel, what should we expect the "sun" to emit? The "opposite" of purple is...yellow?
Now, I'm no astronomer. But unfortunately I don't think a "yellow-light" star actually exists. I mean yes, our own Sun is technically a "yellow dwarf," but I am told that is a misnomer and it actually peaks in bluish light, which we don't properly see due to atmospheric effects. So no, it's not necessarily due to the starlight.
The other opportunity for blue vegetation we could look at is precisely those atmospheric effects. If a planet's atmosphere was somehow made so that the incoming spectrum of light was scattered and filtered to make blue light relatively scarce and a different color relatively abundant, that could plausibly create blue vegetation. Unfortunately this is where my knowledge of physics reaches its straining point, so I'm not going to go in-depth here for fear of giving you wrong information. I can advise you to look at Rayleigh scattering (also known as "why is the sky blue?") but that's as far as I'm willing to reach.
So, are we out of luck? Not yet, not quite. Remember that something is the color that it is because of what wavelength of light it reflects. So, strictly speaking, if you can justify why the plants reflect so much blue light, you can justify their blue existence. Nancy Kiang (author of the article I'm linking at the bottom) postulates that this can be possible around F-type stars, which emit intense blue light – so intense that plants might need to reflect some of it back to avoid getting "sunburned." She cites anthocyanin as a real-world parallel. Anthocyanin is a red pigment responsible for the autumn coloring of leaves, and has been shown to have effects in counteracting UV damage. This makes sense; in particular, Fstars are known for emitting heaps of UV radiation (which is generally not good for sustaining life, but that's another issue you'll have to tackle for your sci-fi).
So there's one possibility I've managed to get ahold of. Your plants are on a planet orbiting an F-type star, which is emitting so much blue light that the plants have developed some reflective component to look blue, possibly coupled with anthocyanin (or an alien equivalent) to give them a more purple tint.
Here are the articles I referenced - they might be of good use to you:
https://www.nature.com/news/2007/070409 ... 409-7.html
https://www.ebscohost.com/uploads/impor ... c-1033.pdf
Hope that helps! Please don't hesitate to ask follow-ups!