In my most recent abstract painting workshop I noticed that the students had difficulty understanding edges and how to manipulate them, so I thought I might offer up some pointers on the subject.
Understanding edges is critical to being able to develop good composition that includes centers of interest or areas of dominance (I use these two terms interchangeably). An edge is the perimeter area of a color or value shape, and can be classified as either hard or blended (soft). Blended, soft edges tend to recede and hard edges come forward. It makes sense then that blended edges may be toward the perimeter of a piece (away from the center of interest) and hard edges should be used at the center of interest to develop the highest contrast. In landscape painting blended edges can be found more in the background, with hard edges dominating the foreground. Blended edges let your eye move between shapes and colors (passage) where hard edges stop your eye and do not allow visual passage.
My students seemed to resist blending color into color for fear of creating a muddy mess. You should have no fear if you are blending colors within the same family, ie: warms together or cools together. there really is no issue in blending colors from different families or those that are complimentary as long as they are not used in equal measure. I get some amazing blended colors by combining warms and cools together. as well as complimentary colors. Just remember that one of the colors needs to be dominant. If complimentary colors are mixed in equal measure the result will either be a neutral gray or brown. These too can be useful depending upon your intentions for the work.
For blending to occur, both colors need to be wet. You cannot blend colors together if one of them is dried. You can overlay a transparent wash if one of the colors is dry, thus creating a blending of transparent layers. A good way to blend wet colors together is to use a figure-eight motion with the brush between the two colors which gradually blends them together and creates a beautiful, soft edge. I hope this helps take the mystery out of edges!