Where was this belief of dressing in black taken from?

Answer

In the name of Allah, these traditions and other similar ones happen in the communities gradually. Regarding dressing in black, it seems that it is universally agreed that people dress in black when they want to show sadness. Perhaps, this tradition emerged after the fall of Banu Omayyad and Banu Al-Abbas regime; a time where the pressure on the Shiites of Ahlulbait (PBUH) decreased in general. Allah knows best.