I don't think this is possible with Screen Sharing. (That would go against the "spirit" of the Screen Sharing feature, which is to help people troubleshoot a problem or work collaboratively on something.)
This is not a free alternative, but if you use Apple Remote Desktop ($80), you have the option to "curtain" the screen, which not only hides what you're doing, but locks the remote computer against local access. It also has many powerful system maintenance/monitoring features.
As far as saving power, could you simply turn off the local display before you leave the location? You should also set the display to sleep when it's not being used (in the Energy Saver pref pane) -- in that case, it would only be drawing power while you're connected and using it. Also, if you're using one of the new LED Cinema Displays, keep in mind they are energy efficient devices, so the power draw isn't as much of a concern as with standard (non-LED) displays.