Yes blue would be lighter, lilac would be even lighter, but very very pretty. Its hard to find magpie's in Mini rex, because its a nowhere sort of color, its not produced often because you can't use it in a breeding program. I mean the japanese harlequin rex you can cross to a tri-color and get tri-colors, they can work well to put more color on your tri's. Magpies are harder to produce, certain color genes have to be present to get one. You'd most likely have an easier time of finding a blue magpie Harlequin(non rex) if you really loved the look, but I geuss you had your heart set on a Mini-Rex. I just got this BEW Mini-Rex that I'm enchanted with, their mighty cute too!
Jenson, are Magpie's an accepted and showable color in Rex in the UK? I know that they have blue Silvers there, and those aren't reconized here.