I tend to use the colour I want the tattoo to have, and then mix this with a lighter step of my skintone. The older the tattoo is supposed to be, the paler it should be.
Also, a good idea is to glaze over the tattoo when the design is done with your skintone of choice.
Also, for blue, use a dirtier, greener/browner shade of blue, as opposed to those reaching into the purple. So to use the old citadel vernacular: Enchanted Blue over Ultramarine Blue...