I had a problem precisely described here: http://www.webupd8.org/2013/01/ubuntu-lightdm-black-screen-when-using.html, black screen with blinking cursor instead of login screen.
I've switched to
sudo dpkg-reconfigure gdm -- but faced the same problem, now loading process stops on "red-white Ubuntu dots" (without activity under this splash). And to login, I still need to restart display manager: Ctrl+Alt+F1 and
sudo service gdm restart.
How to make Ubuntu to boot normally?