Normal attacks have a little area of effect, and when enemies are close a portion of the damage is spread, much like Scindo. It is very noticeable when using elemental mods, as the visual effect is applied with the damage too.
The Dark Sword and its shorter counterpart the Dark Dagger were previously physical damage weapons. Update 13.0 changed them to deal purely Radiation damage on their normal attacks, making them the first melee weapons to do so.
Prior to Update 19, the Dark Dagger and the Dark Sword used exactly the same image for the Arsenal and Codex.