Hi, what causes the phone camera to go black is because the camera of the phone is Faulty,that's why it's not working normal. You will need to replace the camera of the phone. An after replacing the camera,the phone camera will work properly again.
camera prob. some camera prob' caused by lack of memory,
camera damage, comera in not connected properly' cause by bending, drop.
some L7 camera prob.' is on the program of the phone.
If there is technician nearby' try to change your phone camera.