dark
暝 centers on darkness, specifically the dimming of light as day ends or a space becomes shadowed.
暝 combines 日 (sun) with 冥 (dark, obscure), suggesting the sun becoming dark or the darkness that follows sunset.
The sun 日 is beside 冥, a shape associated with darkness. Picture the sun being swallowed by darkness as night falls.
For メイ, imagine the dark of a moonless May evening: May -> メイ, and the sun has set into darkness.