For years now Apple has made transparency a part of the macOS interface which has the effect of blending the menu bar into the background and making menus and some windows take on the background hue as you can see on the left side of the illustration below. For many people transparency blurs the interface making it harder to differentiate interface elements from the wallpaper. It also causes problems for screenshots meant for publication because the images end up with unrepresentative color levels.
To prevent that from happening, open System Preferences > Accessibility > Display and select Reduce Transparency. It can be a significant difference as you can see on the right side of the illustration below.
(Featured image by Adam Engst)