Why is my screen black while viewing an RDP/Horizon console session via VNC?

Because VNC cannot display accelerated regions of the display that bypass the O/S' usual pixel rendering and interact directly with hardware instead. This is true on every platform, including Windows. If you start a VNC server on a Windows machine, run a DirectX application, and then connect to the display from a remote VNC client, you will see the same thing (a black rectangle in place of the hardware accelerated display region).

The only solution, assuming that you want to shadow an existing console session rather than just run a remote desktop, is to turn off hardware accelerated video on the source display. On TLXOS, we use Subtype options to provide this option for modes that typically use hardware acceleration (currently RDP and Horizon). In TMS or Tlxconfig, set the Subtype in the Mode window / Application tab to "RemoteFX (software only)" for RDP, or "BLAST (no H.264)" for Horizon. This will force software-only decoding on your device. Consequently, your remote desktop will run slower, but it will be visible in a remote VNC session.

Historically we used to use hardware decoding for HDX mode also, which is why there is still an "H.264 (software only)" Subtype option for HDX mode.  However, in practice this is only relevant for TLXOS 4.8.x, since H.264 mode is broken/unimplemented in later TLXOS versions (mostly because Citrix's Linux Platform Optimization SDK example code for the Raspberry Pi no longer works when using Workspace App); the default for all new versions is JPEG, which works almost as well as hardware H264 anyway (unless using Receiver rather than Workspace App).