Black screen with cursor instead of login window
Ok guys - I had the same issue with my 2008 iMac and Apple support just walked me through the fix...


Shut the computer down by holding the power button for 10 seconds
Restart the computer and press shift at the same time until you see the progress bar start moving
Once you reach the point where your screen goes dark and you see the cursor, type the first letter of the username for your computer, then hit Tab, then type your password, then hit Enter.
After a moment, you should see the spinning beach ball
Your screen should then move on to something along the lines of "Completing OSX Installation" (I can't remember the exact wording).  Let it finish.  After that it should go to your normal desktop and the issue should be fixed. (If your screen goes pitch black during this process, hit the space bar.  I thought it wasn't working, but the screen was just sleeping).


Apparently it's an issue with an automatic update that never quite got past the login screen.

Hope this helps!