Most of us shade around the eye and head. It helps the eye look a little more natural. I usually shade down the spine as well. if that was my bait, I would add some black to the blue I was already using and shade with that. I don't like using straight black to shade. You can add a drop of black, paint one pass down the side, add a drop of black, etc to help fade the color to a darker blue.
10 lake records, 3 all tackle, and I only added this because Rick did and I have more.