PowerPoint 2010 has introduced new hardware acceleration options for displaying your images during a presentation. Vision is unable to capture these hardware-accelerated images during the demo or remote control session, so the area will either appear as a black box or as the last screen that it was able to capture.
To resolve this with PowerPoint 2010, you just need to turn these new hardware acceleration features off (on the teacher to resolve demo issues; the student computers to resolve remote control issues). To do so, go to file then select options. Go to the advanced section in the options. Check the box labeled 'Disable hardware graphics acceleration' and hit ok. Restart PowerPoint. After these changes it should allow you to view these images during remote control or demo.
Note: Vision Pro or version 8.0 users should not be required to use this option if they are running their screen sharing in Hardware Accelerated Mode.