The main thing is that you need a lighter touch and the right brush size - a fluffy brush is actually too soft for precise blending when you're starting out. What works better is using a smaller, slightly denser blending brush (something like a pencil or tapered brush) and working in windshield wiper motions along the edges where your colors meet. Don't try to blend everything at once; instead, use short, light strokes that go back and forth across that harsh line, and only work in the transition zone between the two shades. This gives you more control than a big fluffy brush that's picking up color from everywhere.
The muddying problem happens because you're probably using too much pigment or blending too aggressively. Try this instead: after you've applied your darker shade, pick up a tiny bit of the lighter shade on your blending brush and feather it into the edge, rather than trying to drag the dark color into the light. You could also grab a clean, dry brush and just buff along the line without any new color - sometimes that's enough to soften things. And let your shadows dry completely between layers, otherwise they just turn into mud.
One more thing that actually helps a lot: practice on just two colors first before you add a third. Get comfortable with blending a simple transition, then build from there. It's about patience and light pressure rather than having the perfect brush or technique.