Radial Blur Bleeding Through Selection

That does NOT work. Please try it yourself with a simple example and you will demonstrate to yourself that lifting to a new layer still leads to blur bleeding over the area. Put a black square inside a bigger white square and blur it and you will see what happens. A MESS

As a rule of thumb, I like to separate the components into their own layers.  You need to make the selection anyway, so make use of it.  This also allows you to reload the selection easily by Ctrl clicking the relevant layer.  However you do have options.

If you want to work on a single layer and blur a selection, then the lens filters do that without expanding outside the selection.  In this case Path Blur does the trick.  I can't show you the Layers panel in this screen shot, but I have Full Image copy selected, with and accurate selection of the sky.

That OKs as this.

In any composite situation, it usually works best to layer up and overlap so that the background extends under the composited layers.

So one of the buildings layers is moved to the top of the stack (turned off for now), and the sky layer selected.

The selection is expanded (Select > Modify > Expand)

I've then Ctrl Alt clicked on the buildings layer thumbnail to leave this band.  It is actually a good idea to expand this selection a few pixels again, because we are now going to use Content Aware Fill to expand the sky beneath the buildings, and you will get edge artefacts as things stand.  They would be easily fixable with the Healing brush, but why give yourself the trouble?

You now have an expanded sky that you can use Motion blur on or do what you like, and the building layer will cover any thinning of the border

This is what I got from motion blur, and it reveals that by CAF was not nearly big enough.  You could copy the layer a few times to firm it up, but it wouldn't fix it completely, so I should have been bolder with the CAF. 

I could have used CAF on all the transparent pixels.  The reason I didn't was this image was 6000 pixels wide, and my system was lagging.  I have downsized to 2000 pixels to do this.    You could certainly use motion or radial blur now.

But the bottom line is that the Lens Blur filters make selection edge bleed a non-problem, so why would you use anything else.

Spin Blur is a good case in point.  Do you remember all those problems you've had with radial blur on things like propellers and wheels.  I used to copy the area to a Smart Object layer so I could open in a new window and easily get the rotation point in the right place, but you still had problems with ellipses.  Spin blur fixes all the problems, and has some more cool tricks into the bargain.

This one by Steve Caplin is good.  Check out his How to Cheat at Photoshop site, and then on the Movies and Updates tabs.