薄暗い is an adjective meaning 'dim' or 'gloomy', whereas 薄暗闇 is a noun referring to the state of semi-darkness.
Compound of 薄 (usu, 'thin' or 'faint') and 暗闇 (kurayami, 'darkness'). The reading うすくらやみ is a straightforward combination of the readings of the components.