Hi, first of all thanks to everyone keeping this alive.
I recently upgraded to Mojave after having El Capitan installed for more than 2 years without any serious problems, but it was time to upgrade... Install went smooth but I'm having same issues with black screen as @tasteTheBacon . His solution to remove all configs and use default config.plist worked and now I can boot fine, but there are some problems: brightness adjustment doesn't work - I guess it will work with the 1600x900 mojave config.plist if the booting problem is fixed. I've also noticed that I'm sometimes getting system freezes for few seconds if I click on some items in the menu bar (Apple logo for example) or when I use Preview app or similar image viewers (Xee^3), it is very random and difficult to reproduce as it didn't happen during writing of this post when I tried to randomly click on items in the menu bar. Is anyone else having same problems? I might try to downgrade to High Sierra, but dark mode feature is great.
Also my T420 configuration is: i5 2520M CPU, 1600x900 resolution, 16GB RAM if that helps.
Edit: Now it froze completely and had to reboot when I tried to open video with Quicktime, also now I've noticed that I can move cursor around while it is frozen.