How do I blur a precisely defined selection and nothing else?

I found a way to solve this problem. This is a focus-stacked composition of 207 images. I pasted a near-focused image and used masking to use the far background from that image, and pasted a far-focused image and used masking to use the near background from that image.  I'll try the Blur filter ideas from the response to learn more ways to use masks.

Thank you for your help.

Phil

If you want lens blur, use the filter... Lens Blur... Using the mask as a depth map will prevent bleeding of subject blur into the background. Black in your mask will hide the blur. In the white you could draw a gradient, for example, to make the depth more interesting. Then go into the filter and pick the mask as depth map.
(To be extra clear: if you would first blur the whole pic, then put a mask over the subject, the subject blur will be bleeding into the background. There's always some problem that Lens Blur... will prevent.)

Do you still have the selection, can you save it (Save Selection...)? If that's the selection intact that you want blur, use one of the Blur filters on that selected area. If the opposite of the selection, you can invert the selection (See Selection Menu options: Inverse).