To get really nice gradients, you have to work while the ink is still wet. So, you need to work a bit faster than usual, and work on small areas, slowly building up to cover the whole section/page. Don't lay down color on the whole piece at once and expect things to be blendable later! I'd also suggest saturating the paper with ink to make sure you don't get a lot of streaks.
The Colorless blender DOES NOT help blend two colors together; it helps blend your marker ink into the color of the paper(use it to fade out the edges of a gradient). To blend colors, you need to work over the area with the colored markers.
Lastly,for cheaper, smooth marker paper, I recommend card stock. It's good if you're not sure about investing a lot into marker art yet, but I like it for my regular work, too. Doesn't need too much ink to become saturated=markers last longer
Here's a good video for lots of interesting tips! She covers a lot of what I've mentioned in my paragraphs up there^