Blurring an image with a regular selection tool will sharply break the boundaries of the selected area.

I will show you how to blur the illustration smoothly in gradation as shown in the image below.

The image on the left is blurred by specifying the selection with the normal selection tool.

On the right is a blurred image created by creating a selection with a quick mask.

Prepare the image you want to blur and then start Quick Mask.

Selection → Quick mask

A quick mask layer has been added.

Next, use the airbrush tool to paint the part of the illustration you want to blur.

The red part is the part that is quick masked.

Release the quick mask when you have finished painting the area you want to blur with the airbrush.

Selection → Quick mask

The selection range is displayed as a dotted line as shown in the image below, but the quick masked part is actually selected in a gradation pattern.

Blur the image.

Select Filter → Gaussian Blur to blur.

done! Blurred smoothly!

Quick mask and Gaussian blur are very useful tools, so it's worth remembering.

Thank you for reading to the end!