Why was Guts called the Black Swordsman?
Guts was known as the "Swordsman clad in black", which was eventually shortened to "Black Swordsman".


Guts before he got his Berserk Armor.


Guts with his Berserk Armor.

His armor was always black, but the most relevant item was his black cape, which always covered his whole body in black when he wasn't fighting.