This is the 12817th most frequent Spanish word.
oscuro
Dark.
In this context, 'oscuro' is used as an adjective to describe the physical attribute of the alley being not well lit.
El callejón estaba oscuro y silencioso.
The alley was dark and silent.
Here, 'oscuro' is metaphorically used to describe the tone or nature of the ghost stories as sinister or eerie.
Siempre me asustan las historias oscuras de fantasmas.
I am always scared of dark ghost stories.
In this sentence, 'oscuro' refers metaphorically to the subject being serious, somber, or morally challenging.