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.
The following settings can be adjusted from the context toolbar:
Width—the brush (stroke) size in pixels. Type directly in the text box or drag the pop-up slider to set the value.
Opacity—how see through the brush is. Type directly in the text box or drag the pop-up slider to set the value.
Flow—how fast the brush effect is applied (1% is very slow, 100% is immediate). Type directly in the text box or drag the pop-up slider to set the value.
Hardness—how hard the edges of the brush are. The brush appears softer as the percentage decreases. Type directly in the text box or drag the pop-up slider to set the value.
More—click to display the Brushes dialog to access advanced brush settings.
Force pressure to control size—Click to control brush size with pressure if using a pressure-sensitive device. This overrides brush defaults.
Stabiliser—enables stroke stabilisation using either a Rope stabiliser or Window stabiliser mode; the former drags the stroke end by a 'rope' to smooth the stroke but lets you introduce sharp corners at increasing rope Length (radius) values by redirecting the slackened rope; the latter will smooth the stroke by averaging sampled input positions within a Window whose size is configurable.
Symmetry—when set to greater than 0, repeats the brush stroke around a number of axes (defined by the symmetry value). The centre axis point can be repositioned by click-dragging it.
Mirror—with symmetry enabled, causes brush strokes to be mirrored along the X and Y axis.
Lock—when checked, prevents the symmetry line from being moved.