With fresh Elementary OS (Juno), I installed VLC media player, a popular free and open source multimedia player.
sudo apt-get update
sudo apt-get install vlc
When I tried playing a video in VLC, I only see black screen. But playing the video works in Audience, the default Elementary OS video player.
To fix the black screen problem when playing VLC player, go to Tools → Preferences → Video and set the output to X11 video ouput (XCB).
X11 video ouput is a render extension that supports blending and scaling for X Window System.