Not available with Hard Edges.
Use Blur to blur the area under the brush.
Blur has soft edges. The pixels closer to the center of the brush are affected more than those around the edge.
Shortcut
Paint Window Control Panel; Tools:
Blur an Area of the Picture
Click the Blur icon
.
Change the line width and style if desired by selecting a Brush Shape, Size and Angle.
Click and drag across the work area from the desired start point to the desired end point. The detail in that part of the picture is slightly blurred. Lift the pen or release the mouse button to complete the action. Each time you drag and release a new area will be affected, until another feature is selected.