Zack Snyder confirmed years ago, through a deleted scene, that Superman was originally meant to don his black suit from the comics in Justice League. However, after Snyder left, this was cut out altogether with Superman returning in his classic red and blue costume. It seems that Superman’s black suit will have a significant part in this cut of the film, to the point that merchandise has been released for it.
As part of the marketing push for Zack Snyder’s Justice League, the official Justice Con YouTube channel has released this clip in HD with no clear differences from when it was shown in July.
The significance of this costume is that it is what Superman wears in the comics after returning from his supposed death. The black suit actually allows Superman to absorb sunlight more efficiently. It acts as an aid, helping recover when he was on the brink of death. Symbolically, it’s also a transition period for the character for when he can return to his true colors and return to saving the day properly again.