"Blending" is a catchall term, the goal of which should be self-evident. You can achieve a blend (i.e. smooth color transition) in a number of different ways. Generally, though, people talking about blending are referring to either wet-blending or glazing.
A glaze, in essence, is a translucent layer of pigment. It can be ink or it can be dilute paint. Successive glazes can be used to gradually build up opacity. One or a few general applications will tint the underlying color of a surface. Apply many glazes of one color over another, decreasing the area of coverage with each successive pass (essentially, layering with translucent paint), and you can build up a smooth transition between the base color and the top color. Here's
an article that goes into more detail.
Darefsky's blending method certainly works, but it isn't what most people (from what I've seen, at least) are referring to when they talk about "wet blending" (which I like to hyphenate - many don't). The other method revolves around working wet paint into wet paint, letting them mix directly on the model's surface. It's a rather fluid [rimshot] process, in which you continually work the various colors back and forth until satisfied. It can be tricky to get the hang of (one of the most cited stumbling blocks for painters looking to reach that elusive "next level"), but it's capable of achieving incredibly smooth blends, and quickly. Thinned paint and/or drying retarder is recommended, so you don't have to rush or risk your paints drying. Many painters have trouble scaling down the technique, though, and reserve it for bigger blends, like those found on broad armor plates, cloaks, banners, etc. It's a pretty hot topic, so search Youtube for video tutorials, if you want a more concrete, visual example.
