Do you need recipe for icing or just how to make it light blue? If you need an icing recipe i need to know what your using it for, there are different types of icing. If your just trying to color already made icing use a half drop of blue food coloring or you can even use blueberry juice.
if you start with white icing, add a drop or two of food coloring, and stir. If its not blue enough, add another drop of food coloring, and stir again. Repeat until you reach the color you want :)
i would say just add a little bit of blue food coloring