I think blending with markers is hard because they tend to dry so quickly. Here are a few tips.
The type of paper matters. There is a special coated paper that works particularly well with markers. Unfortunately, I can't think of the name of it but
www.cornishheritagefarms.com sells it. There is a stamp company that makes incredible art with markers and the paper it sells but I'm having a hard time remembering it. I'll go search and come back if I find it!!
Always start with the lightest shade first.
Also, watercolor paper helps. Stamp the image with waterproof ink and wait a while until it is really dry. experiment to see how long it takes. Then brush the entire image with water and then use the markers onto the wet paper. If you wait until a color dries and then use the marker again, you won't get a gradual blend, but more contrast.
Try making a palette with the markers instead of using them directly on paper. Wax paper, a plastic plate, etc. will work. Rub the marker on the palette and use a blender pen or watercolor brush to pick up the "paint" made by the marker and paint onto the image.