Black Armor is the strongest body armor in the game, but it's not normally obtainable by the player without editing or using cheats.
It also seems to have a somewhat buggy behavior in which the neckline does not always line up with the head. This is most likely a result of it never being completely finished due to being cut from the game.
Black Armor was used by the early Dark Knights before they were removed from the game. Their possession of this extremely high quality armor was one of the reasons they were so difficult to defeat.
The only Black Armor piece weaker than other armor pieces of the same type is the helm, which is outmatched by the Full Helm, Great Helmet, and the Winged Great Helmet. Additionally, a Black Armor piece for hands simply doesn't exist, leaving Gauntlets uncontested.
Black Armor reappears in With Fire & Sword as 17th century style armor.