I had a similar problem when I was trying to install Windows 10.
Windows was installing fine over VNC, but as soon as I changed to my graphics card it booted up, but I only got a black screen.
I tried to install the graphics drivers over VNC, and it looked fine, but never really installed properly.
So my solution was to setup RDP, then edit my VM and connect my graphics card and reboot.
Now it starts up with a black screen same as before.
Next step is to login through RDP from a different computer, and from there install the proper graphics drivers and reboot.
Hope this helps