I recently experienced Black Screen of Death (tm) on my Linux Mint guest after smashing my keyboard (lol). My host is also Linux Mint. I will talk about two potential causes -- incorrect terminal and incorrect login script. There are many more causes such as fsck and video card drivers but these are simple to rule out.
Incorrect TerminalWhat happened was I hit Ctrl-Alt-F2. This switches to the second text console, both on the host and the guest. When I rebooted on the guest, everything appeared normal yet when I logged in it was just a Black Screen.
To fix this, change the host key in VirtualBox under File - Preferences - Virtual Machine. Make it another key than Right-Ctrl. This way the host machine won't capture the key combination.
Next login to the guest machine. Press Host-Ctrl-F7 , Host being the key you mapped above. (Hint if you are not in full screen mode the host key is displayed on the bottom right). If you see your desktop then your problem is solved.
Incorrect Login ScriptThe symptoms are the same -- black screen after logging in. Reboot, and on the login prompt click the gear then select MATE.
When you login, it will say that your default login is some "script". Change this to MATE. Login and you will see your desktop.
Give me Linux BSOD over Windows BSOD anyday.