Confirm that picture from e.g. your blu-ray player is showing when the HDMI cable is connected to e.g. your TV. Then switch the HDMI cable to the "HDMI in" of the Vu+ Uno 4k SE (and naturally, then switch the TV's image to the Uno 4k SE).
Push the blue button from the Vu+ remote control and select HDMI in or the HDMI full screen. Result is no picture from HDMI, just black screen.
Tested with Sony blu-ray player: BDP-S1700B