Nnbveh

So, if i understand it, i have to make a selection, create ad adjustment layer from the selection, apply curves adjustment to it. Frankly, i can't even figure out how to create an adjustment layer from selection...this app is a bit confusing. AH, it does it automatically! Cool. Figured it out.
 
 

Affinity Photo is not Photoshop. Sometimes you will get a better result with a different approach. You can do it the same way in AP but the outcome looks less 'blended'.
As Alfred suggested when making the selection just set the feather to around 20 px. Add the Curves and the result looks pretty much like you finished result. Benefit too, you can paint with soft brush (black/white) on the selection layer to further blend if you need to. No need to 'blur' the face either so saves a step in the process.

Blurring the border pixels is not the optimal way. Better would do the manipulation to a separate layer and feather the border of the manipulated layer. (That is regardless you use AP or PS.)
You could also manipulate original layer direct but then you should start with feathered selection.

In my example you are only feathering the curves mask layer that lets the pixels show through, not the original pixels, so non destructive. Unticking curves layer restores original photo.

Is it just my imagination or is there a child's face in that picture and if so where is it being reflected from?
 
 

I can see it too but I think that's an example of pareidolia.