What I recommend to you is to develop a circular or oval pencil stroke. We learn to color and draw as a kid by zig-zagging the pencil back and forth, making a solid color. With soft-lead pencils, this technique can can create stick points and sharp borders to the pencil stroke, making it harder to blend.
Blending colors is an art that requires you to randomize the shape and direction of your pencil strokes. You may draw the initial area of a gradation with one big gesture (like the blue above), but to make it a smooth and perfect gradation, you'll need to work at it with many, many layers of lightly applied color, slowly building up a smooth, blended tone.
Take your time, be patient. You will not make a good gradation with a few pencil strokes! It takes time to work the surface over and over until you come to a place where you want it. Make your oval strokes up and down for a while, then rotate the page and make our stokes go side-to-side. Then make diagonal strokes. Use a very light touch so that the color builds up slowly.