After upgrading to Mojave I experienced this and have solved it, at least for me.
The computer build is here: https://builds.gg/icedterminal/multibox-16736
The monitor I use is here: https://www.newegg.com/Product/Product.aspx?Item=N82E16824260491
EFI Clover source is here: https://github.com/icedterminal/ga-z270x-ug/tree/master/SSD_EFI/CLOVER
I was originally using 17,1. This worked and I thought at first to just leave it be. "If it ain't broke don't 'fix' it" is what I prefer to live by. I've burned myself before by not heeding that advice. Threw caution to the wind and tried the 18,x versions. 18,3 seems to work best at this time. I tried many things until I ended up with this config: https://github.com/icedterminal/ga-z270x-ug/blob/master/SSD_EFI/CLOVER/config-183.plist
I was able to change the following to get the desired no-flash results.
InjectIntel to NO
Remove the value in ig-platform-id leaving it blank.
Disable both black screen patch variations
Boot arg igfxrst=1
Cleared kext cache and nvram with terminal.
I downloaded the latest Clover installer and selected the following options:
Mounted my EFI and checked to ensure everything was good and removed the other unnecessary folders and files Clover creates. I clicked restart and during boot the screen simply went black and came back just as it did with High Sierra.