Generally, poor network conditions can manifest as black streams. Please follow the below listed actions to troubleshoot this issue.
1. Please check whether your network and connection status are stable. If not, switch to a more stable network.
2. Check if you're able to reproduce this issue on our Sample App. If not, this may be an implementation issue on your end.
3. Check that your browser has been granted the requisite permission to access your camera for that site.
4. Check that you have not accidentally muted the video stream.
5. Update to the latest SDK version.
Please include the following information: