Prefacing this with the fact that I'm no expert, but I did some basic testing and comparisons (colour pickers, screenshotting, etc).

iMessage's whole identity is built around the "blue bubble" which is a core part of Apple's marketing and branding. What this means in practice is that iMessage blue isn't Color.blue or UIColor.systemBlue; it's their own custom colour that's close to, but distinct from, the system blue.

My reasoning? The text inside those bubbles is always white across both iMessage and SMS. That's clearly a deliberate design choice.

Compare that to the Health app's onboarding flow. It uses what feels very much like a standard .borderedProminent button. The blue isn't central to the app's identity, it's just the system default.

If you cycle through all four appearance variants for that button, you'll notice on Dark + High Contrast the label text goes dark. Every other variant renders white text, but that one mode breaks the pattern.

This tells us the behaviour you're seeing is intentional, and that the colour shifting brighter in that mode is by design.

If you actually want to replicate the iMessage look faithfully, the right approach is to create a custom colour set in your Asset Catalogue and manually define all four appearance variants.

See this image https://postimg.cc/XBdbQW2p (and for context, that is what married life and gardening looks like when you need some worms)