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

[pstraus141]

I have a very large image of an irregularly shaped seed pod. I have carefully selected the background so I can darken it and change its color. That's in a curves adjustment layer.

I want to blur the entire background and nothing else. Is there a way to do that using the selection I've already made? 

The file size is 1.7 GB.

Thanks,

Phil

[TheDigitalDog]

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). 

[pstraus141]

I do have the selection, and this is the idea I am after. Thank you. However, I don't know how to apply a Blur filter to the selected area, and not the whole image. How do I do that?

[TheDigitalDog]

I do have the selection, and this is the idea I am after. Thank you. However, I don't know how to apply a Blur filter to the selected area, and not the whole image. How do I do that?

By virtue of a selection of pixels (which may or may not be on one layer), you are controlling what you can do to edit that selection. So if you already have a selection that you used for altering the color via curves, that selection can now be used to blur those selected pixels using any number of options (like blur filter(s). 

[Zesty_wanderlust15A7]

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.)

[Trevor.Dennis]

Easily the best aspect of Lens blur.  

[pstraus141]

Thank you for your ideas. Please explain how to use the mask as a depth mask. 

[Zesty_wanderlust15A7]

I have already explained it. If you would have tried it, it's the "first word" you'll see, unless your PS is in a different language?  You can even invert the mask if you did it wrong, and play with some settings, add a touch of noise, etc.

[pstraus141]

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

[Zesty_wanderlust15A7]

Neural filters > Depth Blur can be cool for quickies. It often guesses it right. However, the edges can be very bad. But you can just export the depth map, work on it, then use that in Lens Blur... Watch a video tutorial or two. PiXimperfect adresses everything I've mentioned.