Take the previous image and start up the mask tool again, and apply the original sky
mask to the NewSky image. You do that by clicking on the icon in the mask tool called
"Combine Masks" (bottom row, next to the last on the right). That brings up a list of masks
you can apply to the image (and you have only the one you made), so accept that and
click "Apply" (below the icons, on the left). Your NewSky image should turn pink again
where the mask is white, over the original bad-sky.
Now we want to feather the mask to make it grow just a bit. Select the feather tool on
the mask dialog (middle of bottom row), set the feather width to one pixel, and "Apply" twice
(at least that's what I did).
Now, click on the NewSky image, and then start the Composite Transformation again. This time
it will start with your feathered mask already applied to the input image (NewSky). Put the good
sky image in as overlay again, but change the "Operation" to "Darken". Now, when you apply
this trtansform, any darker (blue sky) pixels that are on top of the white halo will overlay
the halo and remove it (or at least tone it down).
On this screen shot, I have intentionally put the white amount slider to zero so that it shows
the halo along the rooftop. In the next image, I put the white slider to 100% to show the
effect of the darken operation for removing the halo.