Regarding wearing black clothes.
Query 21: Is the wearing of black clothing regarded permissible? In other words, can one wear full black from head to toe?
The Verdict: It is fine to wear full black, but it should not be worn during the month of Muharram, be it full black or other black clothing, as this is the way of the shias and we should not imitate them. واللہ تعالیٰ اعلم
References
Fatawa Mustafawiyah, Ghaus-ul-Waqt Mufti e Azam e Hind Mawlana Mustafa Rida Khan Qadri RadiyAllahu Anhu.
[Translation]
Mufti Afthab Cassim Qadri Ridawi Noori