A few types of excellent primers that I recommend are the Smashbox Photo Finish Primer (Light formula), which will run you $36 at Sephora but is absolutely worth it. It's a lightweight silicone based clear gel that glides onto the skin and reduces oil. On the drugstore end, the Maybelline Master Prime series is fantastic. There's a variety of formulations on this one, too: Blur and Redness Control, Blur and Illuminate, and a third, Blur and Smooth. These are priced at $9.99 at drugstores and at Ulta, and are lightweight creams that do not add any color to the skin whatsoever. The Blur and Redness Control is a green color but this disappears into the skin as you blend it in, controlling any red pigmentation on the skin's surface. The Blur and Illuminate gives the skin a glow before applying foundation, and the Blur and Smooth fills in pores or fine lines. Again, neither of these leave the face with any added color.
My best tips for blending are to use like with like as far as textures are concerned (powder with powder, cream with cream, for example), as this will ensure the shades you use will appear seamless once blended. As for the blending itself, just keep working at it, buffing the color in circles until one color fades seamlessly into the other and you can't see where one color starts and the other ends. If you find any hard edges are still left behind after you apply, there's a few things you can do to fix it. For eyes, use a clean eye blending brush and keep working the color in circular motions, or gently run a Q Tip where the edge is to soften. For face makeup, a clean, flat head foundation brush can take any edges off.
If you're still a novice at contouring, I would stick to powder products for this, as they blend more easily than liquids or creams. Suggestions I have are pressed powder bronzers or blushes (The Wet 'N Wild Color Icon line is great for this, and costs $2.99 at drugstores), or to use a pressed powder palette designed specifically for contouring and highlighting (A high end option is the Kat Von D Shade and Light Contour Palette, which is also $36 at Sephora. A cheaper option is the Coastal Scents 6 Contour Blush palette, which is $18.95 through their website).
When applying these, use a small headed blush brush (densely packed bristles), or an angled face brush with a small head (same style of bristles) and use a small amount of the product to begin, as it is much easier to add product if needed as you blend, than to start with too much and have to tone it down. Use circular motions here as well, following the natural contour of the cheekbones (if you're unsure, look straight ahead in a mirror and suck in your cheeks, you can ask your client to do so too, as this positioning is different on everyone), and on the jawline. You want to keep blending these products so that no hard edges remain, but these areas of the face are clearly defined.
I sincerely hope this helps you, and remember that practice makes perfect. Good luck!