Comment 1 for bug 1315369

What happens if you move the mouse on the external monitor?
Does the login prompt appear?
Or, are you able to just type (without seeing anything) and hit enter to get in?

The blank screen thing doesn't seem an unity issue, though. Since if you can see the ubuntu logo on the laptop screen, then it means that unity is not blanking the screen anymore, but it looks like that a lower level component (unity-settings-daemon's power module?) didn't turn that monitor on.