I'm surprised no one has pinpointed the probable reason the blue hand is that way. #1 in art, there are dozens of colors to render flesh with, and when it comes to shadowing, and rendering depth, blue is sometimes used as a shade color. Many landscape artists use blues in their shading. Look at the feet. They're not complete or perfect either.
But even more than that is that the artist is probably just a guitar player, and this is clearly mimicking the left hand on a guitar neck. His/her ode to rock and roll guitaring...that's all. Man, I saw that right off. It's so clear.
Also, I'm an artist via illustration and computer, and I have definitely put words into my designs where they are hard to notice, but they are there. It's like leaving my mark as an artist/designer. It puts my thumbprint on it.