-they were darkening
The word oscurecían is the imperfect form of oscurecer in the third person plural.
See the full oscurecer conjugation


En el horizonte, las nubes se oscurecían rápidamente.
On the horizon, the clouds were darkening rapidly.
Las nubes oscurecían el sol matinal y lloviznaba.
The morning sun was hidden by clouds and a light drizzle.
Cómo sus ojos se nublaban y oscurecían.
As his eyes grew cloudy and dim.
En ocasiones se oscurecían las luces de la pista y la aeronave aterrizaba en total oscuridad.
On occasion the runway lights are deliberately dimmed and the aircraft land in total darkness.
Las cortinas de terciopelo granate oscurecían aun más la habitación.
The maroon velvet drapes made the room even darker.
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