Click the teardrop icon from the side-bar menu, then set your brush to 100% hardness. Place your brush on the part of the image you would like to blur, then drag it to blur the desired section.

1) Click the teardrop icon from the side-bar menu.

2) Right-click to open the brush tool settings, then set the Hardness to 100% by sliding the tab to the right.

NOTE: Ensure that you are on the layer of the image you would like to blur before starting the process.

3) Place your brush where you want to start your blur.

4) Drag the brush across the image.

Outcome

You have blurred the desired section of the image.