Go to the phone Setting>General>Accessibility and turn off the 'Voice-Over' feature. After that, wait for a while and again launch the camera app. The common way to fix iPhone black screen camera issue is to reset the power cycle of the device by pressing the Power (Wake/Sleep) button of the device for a few seconds.
Also try a force restart:
Force restart an iPhone with Face ID
Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the side button. When the Apple logo appears, release the button.
If the above doesn't work-take it to apple and they will fix it for you.