I wasn't really sure how I was going to do it until I cracked open the Haemonculus Covens books and spotted the following:
So I tried it out on one of my Wyches, which I'm calling "Nurses" (more on that later):
And the mandrakes are similarly dark, meant to be killer shadows. As I mentioned before, they're meant to be kind of reverse highlighted, with the deepest parts of them glowing a ghostly blue, and the extremities a shadowy black. I haven't really figured out how to do it easily or quickly, so any advice anyone might have would be helpful.