It’s not darker. It’s grayer. If you use a dark mode you’ll immediately see that to be the case. When you pull down the overlay you’ll see the screen get lighter since gray is a lighter shade than black. If you take a screenshot of the control center, go to your photos, open that photo, then do the brightness change, you’ll see that the brightness follows exactly what you expect.

What you are saying is just not accurate. In regards to knowing how light or dark you want it to be, just think about it before making the change. Do you want it to be half as bright? then drag the brightness bar halfway from where it was.

Sure, Apple could completely change how their control center works, but you’re the first person I’ve ever seen complain about this, and if you complain about it with the words you’ve been using then of course Apple is never going to make the change, because what you’re saying just isn’t accurate. Instead file a bug report and say that “Control Center background color obfuscates what the actual effects of the brightness are going to be when using the app I’m changing brightness for” because that’s what your problem is. If you were using a gray app you wouldn’t see the issue at all.