Combine Transparency with Blur

How easy this is to accomplish depends on whether you want to blur just part of an image or not. To blur an entire image, choose the image, then open the Adjust Image palette, and slide the Sharpness slider to the left. To add a translucency effect on top, create a box the same size as the image, set its Fill to Color, make the colour white, and in the Graphics Inspector use the Opacity slider to get the degree of translucency you like (you can also add a Shadow, which will define the edges of the box more, although it will also make the whole translucent box darker).

Blurring just a section of an image (as in part of a background), or blurring text, is a bit more complicated. One way to do this is to use the screen capture functionality to grab the section of the image you want blurred -- use Shift-Command-4, and select the section with the crosshair cursor. You will get a file (named "Picture#") on your desktop that has the section copied. Drag it onto the slide, and position it so that it lines up properly with the underlying image or text. You can now use the techniques above to blur the image and overlay a translucent white box.

Do note that just using a white translucent box gets you a long way towards the effect.