"Photo app on the iPhone 7 screen is black and I cannot take a picture. if I put the front camera it works but when I return the rear camera I can see nothing, it just won't take pictures."
While it is sometimes possible that iPhone 7 Plus/7 mismanaging the update to the new version of iOS. However, it is rare that this grid component go wrong but not entirely impossible.
Above all, we should perform a reset as a new iPhone 7 and do not return your old backup because it is also possible that an application makes conflict (does not properly manage the new iOS) and can cause problems. See if the camera works, which would mean that an app is in question.
If nothing works, you can try replacing the rear camera to see if you can take pictures again. If the camera change solves nothing, then it's possible that this is a component on the motherboard that has blown. In this case, go to Apple, especially if it is still under warranty.
If you want to try more before go to Apple store, you also can try some common methods if your iPhone 7 camera won't take pictures below:
Method 1. Restart the iPhone 7 camera App
To force the camera app to close, then open it again.
Method 2. Restart your iPhone 7
When the iPhone 7 Plus/7 won't take pictures, try to restart your phone as well. Press and hold the sleep/wake button, then drag the slider to power off. Wait a minute and open it again.
Method 3. Try third party camera App
You can find many camera app in the App store, just select one of them, download it and try to take some pictures. If the App works fine, try the default camera App again.
Method 4. Update or Downgrade iOS
If it is the system error causes the "iPhone 7 camera not working" problem, you can try to update your iPhone 7 to the latest iOS or downgrade to last iOS version.
Method 5. Restore iPhone 7
After back up your iPhone 7, then try to restore it. And the process will bring your iPhone 7 to its factory default. It can fix almost all the common problems.