Similar to the smudge tool, the blur tool can be used to clean up after the airbrush. It also works very effectively to make skin look smoother and produce a subtle, healthy glow when used with care. As with the smudge tools, make sure that you follow the lines of the object that you're editing to make the editing less conspicuous and the image more natural.
This applies to all tools: apply with caution! There is a tendency to over edit, especially with the blur tool. While blurring is a wonderful way of disguising flaws in an image, too much blur can result in an undesirable loss of image sharpness, and even stick out all the more in an image than the original flaw. There's a balance to be found, one that requires practice and a developing eye for photo editing.