Modified brushes can come in really handy!
Top left: Dynasty Black Gold, Middle: Princeton Mop, Bottom Right: Robert Simmons Expression
Sometimes I want to soften a tiny little edge, like the chin where it connects with the neck, but I still need the line to stay really accurate. When I do, I use the blue-green and gold modified brush on the bottom.
The one in the middle isn't modified but I use it to more aggressively soften edges or make cloud edges a little more whispy. It's there for a comparison. The black and tan one on the top is an example of a brush whose bristles have been cut down a bit too far and is not as effective at blending - this is to be avoided. You want a nice dome shape with a little more length on the bristles such as the blue-green brush on the bottom right.
To make a modified brush I use embroidery scissors to carefully trim the bristles, of a brush that I had retired from use, into an even dome shape. I then carefully rub it on 400 grit sandpaper to soften the all edges of the freshly cut bristles and to also even out the shape. This is an important step, as without the sanding the bristles are quite stiff and it's like trying to soften an edge with nails. Ew! This modification works best when you use a brush that is soft to begin with like a synthetic sable. I like the Robert Simmons Expression Filbert brushes the best for this purpose.
To use the modified brush you need to have plenty of paint on both sides of an edge. Lightly dab, being sure to involve paint from both sides of the edge, being careful to maintain accurate drawing. Be sure to keep the brush clean. If too much paint has been removed in the process just add a little more and carefully work it a bit with the modified brushes.