You can use the Blur Brush Tool to blur hard edges within an image. The effect is cumulative so the more you paint over an area, the more pronounced the blur effect will be. The blur is only applied to pixels on the currently selected layer.
Settings
The following settings can be adjusted from the context toolbar:
Width—the brush (stroke) size in pixels.
Opacity—how see through the brush is.
Flow—how fast the brush effect is applied (1% is very slow, 100% is immediate).
Hardness—how hard the edges of the brush are. The brush appears softer as the percentage decreases.
More—tap to display the Brushes dialog to access advanced brush settings.
Force Pressure—Toggle to control brush size with pressure if using a pressure-sensitive device. This overrides brush defaults.
Symmetry—when the Lines setting is set to greater than 0, repeats the brush stroke around a number of axes (defined by the value). The centre axis point can be repositioned by dragging it.
Mirror—with symmetry enabled, causes brush strokes to be mirrored along the X and Y axis.
Lines—with symmetry enabled, defines the number of axes around which the brush stroke will be repeated.
Locked—when enabled, prevents the symmetry line from being moved.
Blur Brush Tool