For those who don't want to pull the phone apart to fix the sensor alignment, here are three other options beyond what's already been mentioned:
1) Use the Sensor Disabler app to disable to proximity sensor (https://play.google.com/store/apps/details?id=com.mrchandler.disableprox)
That's what I did, but it requires the phone to be rooted and Xposed to be installed. The proximity sensor doesn't work anymore, but I don't really care since I can manually turn off the screen when necessary.
2) Go to Settings > Accessibility > Power button ends call
3) Use a Bluetooth headset/device to end the call
The last two options don't help with the black screen during the call, but they give you control to end it.