Before you begin, if you're having this problem run through this check list:
1. Make sure you're computer is indeed on
2. Make sure you're using an iMac SMBIOS (Mac Pro System Definition doesn't have this problem)
3. If your card requires Web Drivers (GTX 9xx), make sure they're installed and boot with NvidiaWeb=true in Clover.
4. This guide is focused specifically on Nvidia GPUs, but it elements of it almost certainly pertain to AMD GPUs as well.
Advantages of this...
[Fix] 10.12 Nvidia 2/3/4/5/6/7/9xx Series Black Screen
1. Make sure you're computer is indeed on
2. Make sure you're using an iMac SMBIOS (Mac Pro System Definition doesn't have this problem)
3. If your card requires Web Drivers (GTX 9xx), make sure they're installed and boot with NvidiaWeb=true in Clover.
4. This guide is focused specifically on Nvidia GPUs, but it elements of it almost certainly pertain to AMD GPUs as well.
Advantages of this...
[Fix] 10.12 Nvidia 2/3/4/5/6/7/9xx Series Black Screen