The Dark Sword is a very slightly better version of the Silver Sword, giving 400 more +ATK for the exact same scaling. It (unlike the Silver Sword) also cannot be found in shops, which coupled with the middling price tag for its direct competitor makes this weapon generally not worth the time to specifically farm for. However, if you're stuck at about level 5800 it might be worth to fight in the zone it drops in for it is not useless.
In-game stats Edit
Cost: Can't be bought
Category, ID: 0,21
Drops from: Insect Warrior @ LV 5300, Haunted Graveyard