Florent Farges has a nice demonstration of blending. He paints two spheres side by side, one with blending, one with just application of the right color/value in the right place, with a loaded brush. He cites Speed and Sargent as examples to follow. It confirms and vividly demonstrates what Mark is telling us.
Comments
If you place the right color in the right place, and never blend, then muddy color is not even possible.
If you do need to blend, then Mark recommends putting the darker of the two colors on the brush, and make one or two (no more) strokes along the boundary between the two colors. No more.
It is touching the canvas as little as possible that keeps the colors pure.