At its core, the blur test, as designer Lee Munroe explains, is looking at a blurred version of your site and see what elements stand out.
If it’s not what you want to stand out, it’s time to go back and make some revisions. Looking at a blurry version of your page will reveal the bare bones of your visual hierarchy, letting you see the layout with fresh eyes. The actual blurring of the site can be done in Photoshop, by adding a 5-10 px Gaussian blur on a screenshot.