dark, sallow colour
黧 centers on a dark, yellowish-brown complexion or color, describing a sallow, swarthy, or dusky appearance.
黧 combines 黑 (black/dark) with a phonetic component that likely contributed the sound. The character specifically refers to a dark, sallow complexion.
The bottom part 黑 means black or dark. The upper part looks like a person with a tired, yellowish face. Together, 黧 is a dark, sallow complexion.
For レイ, imagine a person with a sallow face laying in the dark: lay -> レイ, and their complexion looks dark and yellowish.