Mac Pro custom graphic black screen during boot
"Custom graphics card" is the problem… or more precisely, PC graphics card, not Mac.

Unless you bought one that was already Mac-flashed you will get nothing until the OS starts up.

The only two options are 


keep your old card around &amp; swap it in if you need to see the boot process, or  
get that one flashed.


More info/opinion that I can reasonably précis in here - http://forums.macrumors.com/threads/the-golden-guide-to-flashing-graphic-cards.877441/

Actually, from that I notice that the Zeus flashing tool is by Oskar Groth, aka Cindori - which ought to make it at least a trustable method
See - https://www.cindori.org/software/