My niece (real life) has the neatest witch mark - a patch of snow white locks in her otherwise jet black hair. I'm very jealous of it, I wish I had hair.

Does anyone know of such hair design so I can make a sim with a witch mark??

Some of ChazyBazzy's older retextures have strands of hair that can be recolored separately:

Female only
Alesso Cliche (long, asymmetrical sideways braid)
Alesso Stone (short)
Cazy Ordinary Day (medium, wavy)
GeishaSims Firefly (short, with bangs)
HA2D 01 (short, asymmetrical)
LeahLillith Afterglow (long, asymmetrical sideways braid)
LeahLillith Heart (short, asymmetrical)
LeahLillith Lush Life (medium, over one eye)
LeahLillith Universe (long, wavy, asymmetrical)
Newsea Azure Sky (long, curly, with bangs)
Newsea Bayou (long, curly, asymmetrical)
Newsea Cambrian (long, ponytail)
Newsea Carly (long, pulled back, asymmetrical)
Newsea Crow (long, with bangs)
Newsea Ferris Wheel (medium, tied back, wavy, with bangs)
Newsea Flying Dance (long, with swept bangs)
Newsea Lucky Star (long, ponytail, with bangs)
Newsea Nightbloom (medium, wavy)
Newsea Sail Away (long, with bangs)
Newsea Uproar (medium, pulled back, asymmetrical)
Newsea Viola (long, ponytail, with bangs)
Nightcrawler 26 (long, wavy, asymmetrical)
Nightcrawler 27 (short, with bangs)
Nightcrawler Confetti (short, asymmetrical)
Nightcrawler Get Up (long)
Nightcrawler Gigi (long, asymmetrical ponytail)
Nightcrawler Pearl (long, asymmetrical sideways braid)
Nightcrawler Sugar (long, pulled back)
Stealthic Sleepwalking (long, wavy, asymmetrical)
Zauma Mirror (short, double buns)

Both genders
Kijiko Abyssinian (short, asymmetrical)
Kijiko Burmese (short)
Kijiko Maine Coon (short, asymmetrical)
Kijiko Toyger (short, asymmetrical)
M1ssduo Toru (short, asymmetrical)
Newsea Battler (short, asymmetrical)
Newsea Black Bullet (short, asymmetrical, wavy, shaved)
Newsea Rough Sketch (short, asymmetrical)

...And a ton of others by Kijiko: http://chazybazzy.tumblr.com/tagged/Mesh%3A-Kijiko

Male only
Cazy Joey (short, asymmetrical)
Kijiko Panda KangKang (short, with bangs)
Newsea Robin (short)

I've never heard it called that before. I know there's some sort of scientific name for it but I can never remember it. But yeah, doesn't it look classy?