That would be the "Increase contrast" setting, that somehow decided to turn itself on during the update. At least in my case that was it. Which I don't mind, I actually like the darker colors.
edit: Now that I think about it, on iOS 11 there was a setting to slightly darken some interface colors (no idea what it was called..) that I had on. Maybe they just renamed that to "Increase contrast" and applied it on a wider scale than previously, like the message bubbles, whereas previously it mostly affected some interface elements like "Back" in menus and such.