young girl's hair style
黰 refers specifically to a young girl's hairstyle, with no broader conceptual center beyond that single meaning.
黰 combines 黑 (black) with 真, likely as a phonetic component. The exact historical development of this specific character is uncertain.
The left side 黑 (black) suggests dark hair, and the right side 真 (truth) can be thought of as a young girl's true, natural hairstyle. Picture a young girl with her hair done in a simple, traditional style, the blackness of her hair showing its natural truth.
For シン, imagine a young girl showing off her new hairstyle: she says 'See? New!' — 'See New' blends into シン, and the scene ties back to the hairstyle meaning.