A black picture does not always mean a network failure. First check whether the source preview has a picture, whether viewers receive video, and whether audio sharing is enabled.
Services such as Netflix and Disney+ may prevent screen capture. If ordinary browser content shares correctly but the film stays black, protected playback may be the cause. Each person should use their own account to watch that service.
In the browser's share picker, select the movie tab and enable tab audio if available. Window and system-audio support varies by browser and operating system. Stop sharing and select the source again if no audio track was included.
Restore a minimized window and keep the shared tab open. For games, try borderless window mode. Share a normal page briefly to check whether the problem affects every source or only the movie.
Reduce other uploads and try a stable network. VPNs and Wi-Fi device isolation can interfere with a direct connection. If the picture stays black, restart sharing and compare the host's preview with the viewer's screen before changing settings.