The black viewfinder issue is caused by some incompatibility:
- You may be using the wrong configuration for your device.
- Some phones, like the OnePlus 3/3T/5/5T, need a fixed .lib file to rectify mistakes made by OEMs.
How to fix the issue?
If your phone needs the camera fix:
The easy fix:
New versions from developers like Arnova8G2 come with an option called "Disable Front Camera HDR" or "Fix Black Screen". Go to the settings page, enable that option and restart the app.
The downside of this is that things like HDR and portrait mode won't work on the front camera.
The proper fix:
Some phones already have a fix available that allow you to use all features (HDR, Portrait, etc) on the front camera:
- OnePlus 3/3T Front Camera Fix
- OnePlus 5/5T Front Camera Fix
- Other phones: Check the XDA subforum for your device to see if a fix is available.
If your phone doesn't need any fix:
You're probably using the wrong settings/configuration (very common with apks with lot's of settings).
For most phones the best "camera config hdr+" is "nexus 6 auto" together with one of Pixel models.
I suggest you visit XDA Developers, go to your phone's forum, look for a Google Camera thread and see which version/settings people are using.