There are several reasons a home winemaker might want to utilize blends. A good example is to blend a wine that is too dry with one that is too sweet to create a more balanced wine. In this instance, the two wines should be similarly based, both Merlots, for example. You may also want to blend wines to create a new flavor profile, integrating complexities from varying bases.
Blending grape and non-grape wines can yield some interesting and delicious results. It is the ultimate way of creating a new flavor profile. In most cases, the grape wine will predominate, both because it brings body to the blend and because fruit flavors seem to blend better with grape than the other way around.
Another way of saying this is that grape wines tend to absorb a fruity character from non-grape wines, but non-grape wines tend to lose their fruitiness to grape wines. With a little experimentation and patience, blending is easy to do.