Adding blue coloring to chocolate icing? That works in small batches for Halloween cookie decorating. For fun try melting some chocolate chips and adding a bit of blue food coloring. Also Betty Crocker makes a ready-made Hot Fudge that's really dark that you might be able to use for the cake or add the Wilton's paste to.

Now if you were making a Kick-worm cake I could help. That's a bundt cake cut in two and shaped lengthwise into an S. Frost with green icing and add licorise for legs, suckers for antennae, and gumdrops and M&M's for other features. It's easy to slice.

Hope someone can help you with the whale. I know how little boys are!