I thought I would try a more detailed explanation. GStreamer cant display black borders for scaled video with KMS. So the console text displays underneath it.
I have tried this but it doesn't blank the screen, just disables cursor and keyboard, and it requires root. My service runs in a system user. It should be able to return back when streaming stops.
See here for more details
https://www.raspberrypi.org/forums/view ... 7&t=253976