Hi,
the black screen when trying a gpu could down to a bios setting. Make sure secure boot isn't enabled and the bios is set to legacy. Look under the 'Peripherals' section and you should see 'Initial display output', this is where you set what the machine uses for graphics.
I doubt that both gpu's are faulty and it's probably down to a setting but just in case is it possible for you to test the cards in a friends machine?