Here same issue on iMac A1418 late 2013. Black screen. Had two times since, a desktop on the primary screen. Since then only black screen again. Heven replaced the Logic board, same issue. Replace LVDS cable by new one, same issue.
Memory taken out and restart and plugged-in again as well.
Since I had two times suddenly a very good normal desktop picture, I don't want to replace the full screen yet.
On the 2nd screen the desktop is perfect, while at the same time, de primary screen shows black in the middle (4:3 format) and next to this area vertical side bars when desktop set to primary screen , by CMD+F1, OR if primary screen is used only, without 2nd screen connected.
SSD formatted and MacOS Catalina newly installed.
Annoying is that some commands for recovery etc. are not shown on the 2nd display...
Error occurred approx. one year after replacing HDD to SSD and added memory to 2x8Gb