Welcome Davide Consiglio,
Since this is a public forum, we will hide your personal identifiable information, and I have sent that image of yours containing your personal identifiable information in a private message so we can work together to resolve the issue.
Thank you for reaching out to Microsoft Q&A. Since the blur only affects some notifications and is gone after Win + Ctrl + Shift + B, the notification banner is probably being drawn with the Windows transparency (blur) effect. Let's narrow it down first and then test a few quick things.
A few quick questions
- Is your PC signed in with a personal Microsoft account, and is it not managed by a work or school organization? Please also tell us the edition (Home or Pro) under Settings > System > About
- Do Windows own notifications (for example from "Settings" or "Windows Security") also look blurry, or only apps like WhatsApp and Chrome?
- Do you use one monitor or several? Is "HDR" turned on for the display?
Things to try now
- Turn off transparency. Press Win + I, go to Accessibility > Visual effects, and set Transparency effects to Off. Then send yourself a few test notifications (for example, a WhatsApp message) and check whether the banner stays sharp.
- Check HDR. Go to Settings > System > Display > HDR. If Use HDR or Auto HDR is on, turn it off and test again. If the display has more than one entry, check each one.
- Check for updates. Go to Settings > Windows Update > Check for updates. Then open Advanced options > Optional updates and install anything listed there. See Get Windows updates as soon as they're available. Restart the PC afterwards.
- Repair system files. Right-click Start and select Terminal (Admin). Run
DISM /Online /Cleanup-Image /RestoreHealthand wait for it to finish. Then run sfc /scannow and restart when it completes.
Looking forward to your update.