When shading large areas to achieve smooth blends you have to take a couple things into consideration, like using a larger mag. It covers a larger surface, when you use a smaller mag it increases the risk of being sketchier, and your blend will not be as smooth.
Another consideration is your motion you have while you're tattooing. If you use a circular motion its going to leave lines, because as you're turning and coming down with the machine the needles line up parallel, so when you're coming down on the circle it will leave a solid line instead of a blend.
You want to use a brush stroke with your machine, and also consider the consistency of your motion. You want to hit the same speed back and forth during the entire tattoo or the are where you're trying to achieve the smooth blend. Have a simple back and forth motion that is really smooth. Do not speed up or slow down. You want to have that same consistent rhythm.
Using a larger mag reduces the time that you're going over an area which leaves less room for error. Basically you get less strokes and cover a wider surface, resulting in the shades looking a lot more smoother. Another thing I like doing to in these big areas is just kind of follow the motion or the shape that you're shading. Follow the motion of any curves with the same motion: back and forth.
You want to make sure too, when you're trying to achieve these smooth blends, is that your machine is running consistent, and it's running fast enough to compensate for the motion that you're doing. If I was tattooing and running the machine at a slower rate and i was moving this fast it would snag on the skin or it would skip, and leave little skip marks I’m sure you guys have seen them. Thats what it is when the needle doesn't have enough time cover that whole area as your working, it'll skip over. You just gotta make sure your at a consistent pace.
Hope these tips help and keep working hard and perfecting your craft. For a more personalized experience, I offer individual virtual tattoo webinars. See more information here.