I saw you have a X99 system without Intel iGPU and featured a AMD RX 480, how do you boot into system without black screen?
I build a X99-A II + E5 2696 v3 + GTX970 system, I install 10.13 public beta, all okay, except no Nvidia driver support, so I buy a new video card RX460 and think it will be good and I can use RX460 until 10.13 and Nvidia release official version, but!! black screen issue!! I know it after black screen happened and googled!
I search a lots, and find no answers, so disappointed ...
Okay, just read it!
You need a helper card because the RX480 cannot be the primary GPU. If it is the primary GPU, the driver crashes when it tries to initialize the card and you get a black screen.
The only workaround at the moment is to set your primary GPU as the the 'helper card' in your BIOS, or if that is not an option, usually a motherboard will make the card in the first PCIE slot the primary. So make sure your RX480 is NOT in this slot, and is in a different one. The goal here is to make the computer/clover use the helper card. You do not need anything plugged into it, but you should have your display plugged into the RX480.
If you did it correctly, you will get NO video on your display from the RX480 until right before the macOS desktop/login screen appears. No BIOS post, no clover, no boot progress bar, nothing. That is because it is sending the video to the 'helper' card because it is primary.
If the RX480 is left untouched/unposted, then when the macOS AMD4100.kext driver loads (right at the end of the boot process), you will magically get accelerated video and the RX480 will work and you can ignore the helper card. If you run into problems during boot, you have to manually plug your monitor into the helper card and see what's up.
It's a less than ideal solution, but its what works for now.
I tried to install 7770 and RX460 ... just keep rebooting... sad