Random Black Screen - Backlight Off - Panel On!
Posting a solution to my own question as I have not seen this answer on here before!
24” Cinema Display - Random Black Screen - Solution
24” Cinema Display, connected to iMac 27” as an external display and it works fine however at random points the screen on the Cinema Display will go black. Checking the screen with a torch I can see the LCD is still on and working as it shows my desktop.
I have hot corners with display sleep turned on so if I move the mouse to the corner the Cinema Display (while already black) properly sleeps, moving the mouse a second later causes the screen to come back on again fully lit.
The time between black screens can be totally random. Sometimes it works for weeks with no problem. Sometimes the black screens return within minutes or even seconds.
Troubleshooting:
Reset PRAM / NVRAM
Reinstall OS - same problem on Sierra, High Sierra and Mojave
Open the Cinema Display and clear out any dust
Reseat all internal cables
None of these fixed the issue. Sometimes they would appear to resolve it but the problem always came back.