My setup (Mac Mini 2018 / Mlackmagic eGPU / LG 5k) shows black screens during boots and reboots. I.e. on 50-60% of all boots and reboots I see the Apple logo, 1 or 2 seconds of status bar and then the screen goes black. If I'm lucky, after 20sec I see the login screen, if not the screen stays black, until I disconnect the TB3 cable from the monitor and reconnect it which then will display the login screen. After this the combination works flawlessly (until the next boot or reboot). For me that sounds like it's related to what is discussed in this thread.
I just found this thread... For the past 2 month I documented my support odyssey in this
thread. (Only strange thing is, that my problems started already with Mojave 10.14.6 when I bought the Blackmagic eGPU)
Now I'm tempted to find a used Blackmagic eGPU Pro to figure out, if this behavior really stops, when I'm using a Vega based eGPU.
And after everything I tried...
_Trying another LG 5k
_Trying different TB3 cables
_Clean installs of Catalina
_Got my eGpu replaced by the Blackmagic support
...it would be somewhat logical, if it's related to MacOS driver issues...
Would be just great if Apple would fix it!