Hello.
I have the following blur related problem.
I have two objects of different color/shade next to each other. I want to select one and blur it, while leaving the other one untouched. However, when I do that the gaussian blur, which is my preferred blur method takes pixels from one object and averages them into the calculation and so the final blur contains pixels from the adjacent area.
So if I have a black and white squares, I select the white square and blur it with gaussian the result is not pure white, some of black pixels get mixed in and the result is a gray patch adjacent to the black square.
Here is what I have tried and it has not worked.
Select and blur inside the selection.
Select, bring onto the new layer, blur inside the selection. Even when I do that the blur reaches onto the layer below and grabs the pixels and averages them in. Bottom layer's visibility is turned off, doesn't matter. It's maddening.
My transparent pixels are locked.

Some other blurs don't do it, but the ones most useful to me, especially gaussian, always does it. Unfortunately, the other blurs are not as easy to "tune" as far as the strength of the effect so gaussian is best for me.

The only thing that has worked is selecting, moving onto new document, blurring, then pasting back onto original document. That takes way too much time on my computer and I can't imagine that there isn't a smart easier way to do it.

Thank you

There is no one size fits all, every image is different.

here is a few things I do
1. surface blur, careful if you work in 16bit, I have a fast machine,
but that can bring it to its knees
2. blur, then fade (command+shift+F) then go to darken,
or lighten  depending on the background
3. use dust and scratch filter, it's similar to surface blur
4. use the history brush to lessen tone contamination

Now sure if I understand your question well. But it seems to be the same question of how to blur background in a portrait photo. If so, I usually do this:
select A, save it as a channel in channel panel.
go back to layers, delete A, now you have the rest, that is, B
blur it,
go to channel panel, bring A into selection,
make it a mask