The black suit is worn by Superman after his resurrection in the comic book storyline The Death of Superman, which many thought would be partly adapted into Justice League.
Justice League was affected by highly publicised reshoots, so was the black suit a victim of the cutting room floor?
Throughout the production of Justice League, little teases were dropped — mostly by Henry Cavill and Zack Snyder — that Superman would be wearing a black colored version of his costume.