When it comes to digital images, colours aren't simply stored as red, green and blue. In fact, they're modified so darker values are stored with more granularity than brighter ones. Unfortunately, if an image editor, such as Photoshop, doesn't take this into consideration when say, blurring, you'll get an incorrect — and sometimes poor-looking — result.
If you're wondering how Photoshop gets it wrong, this video from minutephysics does a great job of explaining. Essentially, most image editors use the stored colour values (that have had their brightness altered), rather than the converted ones, for filtering operations.
Fortunately, there is an easy fix — at least for Photoshop. Hit Shift+Ctrl+K to bring up the "Color Settings" dialog, or find it in the Edit menu. Then, locate the checkbox that says "Blend RGB Colors Using Gamma" and tick it (leave the gamma value itself at "1.00"). You may have to click the "More Options" button.
Here's a screenshot of the dialog window from Photoshop CS5.
There is a caveat. While you'll get "colorimetrically" correct blending, you will notice a difference between Photoshop and other imaging programs, which could cause problems if you use a specific workflow. If you are able to use it, you'll immediately notice the improved quality when blurring or blending.
Computer Color is Broken [YouTube]