You can use layer blending options to change dark areas to transparent. The treshold is adjustable. An example:
A portrait which contains much darkness, it (the layer in Photoshop) is converted to Smart Object:
Gaussian blur filter is applied, a smart filter is generated automatically after clicking OK in the blurring dialog:
Editing the Smart filter content opens the original image in a new window. There Layer > Layer style > Blending options gives a possibility to make a tresholded mix with underlying layer:
Here's no underlying layer, the treshold slider leaves only the bright parts visible, everything else is turned to transparent.
When the content image is saved, the edit is updated to the blurred image:
If Transparency isn't the wanted replacement for the darkness, you can insert a new background layer (here it's grey) to the image in smart filter edit window or under the smart filter layer. In the first case also the new background is blurred, but the difference is invisible if the new background is uniformly colored. Here the new BG is inserted in the Smart filter edit window. The result is this:
Inserting the same BG under the smart filter layer gives this:
Not asked: 1) Gaussian blur doesn't keep your highlights intact. They are blurred too and if they are close enough a transparent area they become weaker (=partially transparent).
2) This was complex. To make it simpler get another program. Krita (freeware) allows you to insert effects as filtering masks to a layer for non-destructive edits. In this case you want to insert Color to Alpha and Gaussian Blur as filtering masks to the same layer.