There are two ways to 'blend' them better. The first: create a volumetric sphere for the atmosphere larger than the planet around the planet that is 'fuzzy' in the materials selection option. The second: add a light with the color selected to a blue that is almost black shining on the dark side of the planet to enhance the shadow area. Ideally, that dark sides should almost disappear unless, because of the planet, they are receiving a colored reflection that is similar to the planet itself. Hope this helps. Mark