Random Black Screen - Backlight Shut Off - Solved!
Answering my own question here simply to provide a solution that I have not seen listed anywhere else before!
Imac 27" 2009 & 24” Cinema Display - Random Black Screen - Solution
iMac 2009 27", computer works fine however at random points the screen 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 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. This happens with the screen being used both directly and in Target Display Mode.
Reset PRAM / NVRAM
Reinstall OS - same problem on Sierra, High Sierra and Mojave
Running the fans at higher speeds
Open the iMac 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.