Blur tool doesnt work?

[AnnN]

Hello,
 selected the boy in the middle to have him separated from background (and two girls in the front, you know what I mean). I then used filter "Depth of field" but now the edges of two girls look too sharp on some parts. I tried to use soft eraser but soon realized that it'd be transparent. I then wanted to blur the edges with the Blur tool, but it doesnt work, it does nothing.
Any suggestions?
Wish you nice evening and thank you

[AnnN]

Okay so if I were in Krita, I'd solve it by clicking on this lock, to allow me to go past the limit, the borders. 

[Ron P.]

Not meaning to be rude with this question, just wanting to understand your knowledge on this.

Are you, and do you have a firm understanding of photography?

The photo that you're trying focus the viewers attention will result in the girls not being as blurry (out of focus). Typical these sort of shots are done with very fast lenses, those having a apertures of f2.8 or less. Not uncommon to use a lens shooting f.1.4, which gives an extremely narrow depth-of-field. Macro lenses are real good for narrow DOF.  DOF is based on the plane the objects occupy. If two objects are in the same plane, as seen by the camera, they will be affected the same. 

Now what I would try on this photo is to use a Lens blur, or DOF blur or even a Gaussian blur. Then invert the blur layer, this hides the effect. Then paint in your blur. You can also try the opposite, do not invert the layer. Use a black brush and paint out the area you do not want out of focus or blurry. It takes some trial and error with the brush settings to find the right settings.

One other thing, the sharpness on the edges of the girls is due to the selection not the blur effect. Have you tried refining the selection? Feathering it? Again you need to experiment with the settings in the refine selection.

Here's a quick example of what I'm trying to get across. The image is a stock image from Pexels. I used a DOF blur, but changed to the Tilt-Shift blur. I uploaded my afphoto file complete with history. This was a 5 min job, so not as good as I would like it.

 

dof_blur.afphoto