I wanted to accomplish the same thing as the original poster and went looking for a property or action within PowerPoint that would work. After reading the previous posts in this thread, a pretty simple solution dawned on me. It works for me, see if this will work for you:
At the point in the presentation where you want the 'blank screen' - that is a black 'nothing screen' (what you would see if you hit the 'B' button on your keyboard)...
Step 1) Insert a slide with nothing on it.
Step 2) Add a rectangle shape to the slide that is the same size as the entire slide (to do this in PowerPoint 2010: 'Insert' > 'Shapes' > 'Rectangle'. Size the rectangle by dragging a diagonal corner and repositioning as appropriate.)
Step 3) Right-click on that newly added shape, and select 'Format Shape'. In 'Fill' > 'Fill Color' select Black and be sure 'Transparency' is set to '0%'.
That's it. Now when you transition to this slide, the screen will appear to be blanked out (although it is really just a slide with a black rectangle filling the whole slide). When you leave this slide, it will appear as though the screen has been "unblanked"
You can copy/paste this slide anywhere in the presentation where you want to speak for a moment and ensure attention is on what you are saying opposed to what's on the screen.