I'm having trouble doing a small Gaussian blur on a photo in a document in P&GD10.

I just need to blur a tiny part of the picture, so I've used the mask painter to cover it, then inverted it so most of the picture is masked, except the bit I have to blur.

Then I apply the Gaussian Blur effect and the whole doggone, blinkety-blank picture is blurred. Geez I'm getting frustrated. I think I'm reading the Help properly. What the heck am I doing wrong?