Original post by: Tom Hinde ,
27" Imac Mid 2010 running El Capitan. Started having the same issue when I updated to Sierra last year. Reverting back to El Capitan solved the issue until this morning, where it started flickering before going completely black. The addition of "SMC Fan Control", and operating the fan speed at higher RPM's brought the screen back. Brightness must be 100%, otherwise it still flickers. It definitely seems like a heat dissipation issue, all things considered.