When you are using Kerberos.io with a USB camera, it's very common that you will see black images, the first time you run the system. This is because Kerberos.io is configured to use the Raspberry Pi camera by default. If you are using a USB camera, you will need to change the capture device first. You can do this by using the web interface; on the settings page.
Make sure that you've attached the USB camera or Raspberry Pi camera module before you've powered on the Raspberry Pi.