Wearing black is a sign of mourning and grief on tragedies of the Prophet (SAWA) and his Progeny Ahlul Bayt (AS).
The Prophet Muhammad (SAWA) himself did wear black as you can read in Saheeh Muslim (Hadeeth number 1359).
Those believers who love the Prophet (SAWA) and his Progeny (As) feel sad on remembering their tragedies hence they wear black to express their noble feelings.
Wassalam.