Using Gaussian Blur to blur the background of an image is not the best way to go.

Let’s look at this image in which I’ve selected the background:

Selected background

After applying Gaussian Blur you’ll notice a dark halo around the main subject:

Dark halo

Look at the following example in which I have selected the right color and applied Gaussian Blur:

Bleeding colors

What happens is that Gaussian Blur allows the colors outside the selection to bleed in, something we wouldn’t expect, because after all most people including me would assume that only the pixels inside the selection would be blurred and that the rest would be ignored.

Adobe introduced in Photoshop CS a new tool called Lens Blur. Not only does it blur in a more natural way, but it also ignores everything outside a selection. Notice how I got rid of the halo and that the main subject really starts to pop out:

Lens blur