If you get blank screen with just a cursor on Ubuntu, probably it’s due to lightdm issues with systems running on SSD.
come out of the blank screen by: CTRL + ALT + F1
use gdm instead of lightdm: sudo apt-get install gdm
if gdm already installed: sudo dpkg-reconfigure gdm
I got full resolution on mac after installing Virtualbox drivers that popped up in Additional Drivers. After that I had to mount the VBOX Guest additions ISO from devices menu and restart for the changes to take effect.