the values and blending on the last skull studies seem right, what pencils are you using because it looks like your using some light then something really dark, id suggest using some H pencils with even pressure then going from like a 4h to a 3-2h then layering over to get nice smoother blends.
great start for your sketchbook dude
i started out with graffiti aswell, dont do much any more though
just keep at it man it all looks promising
looking forward to seeing your progression
have a good one
One advice that may help you to make these studies more interesting.Try different pressure when you draw lines and not always the same thickness.Experiment with different kind of lines and different ways to draw a lines.Keep up with hard work.