When I swipe finger fast and quickly up to half of the dock icons on my iPad Air with the iOS 11.1, it opens the Control Center but it just displays without blurred background. I try a simple Restart and it still occurs. I always need to access the Control Center and that make it to become unreadable. It is very annoying for me and I could not figure out how to get rid of that on my iPad tablet.
Many users have reported the missing background problem on their iPad Mini or iPad Air model. It seems to be bug with the iOS 11 so the blurred function stops working. I have upgraded to iOS 11.1.2 and still face that, and you could try the tips to fix it until Apple release the path.
The simple work-around
* Swipe up more slowly to open the Control Center. For some reason, you might swipe up too quickly and it gets this behavior to be happening.
it also depend on the length of swipe. For a short swipe up, it brings up transparent (non-existent) background. For a long swipe up, the background is more opaque or completely opaque.
Enabling the Reduce Motion feature runs into the issue, and you have to disable it completely to get hardly any background on Control Center.
* Launch the Settings app from the Home screen.
* Tap on General, and tap on Accessibility.
* Select the “Reduce Motion” option and turn the option from On to Off.
As an Accessibility feature, Reduce motion settles down and smooths out the zooms, pans, parallax, and other dynamic elements on the iPhone and iPad. I prefer it enabled. After turned off Reduce Motion, the motion becomes slow and not smooth on iPad mini
Another method is to downgrade to the iOS 10 on your iPad.