To get a free Dark Blade in Blox Fruit, you can receive it as a gift from a friend who already owns it. Another method is to trade items like Pimp Fruits with other players who have the Dark Blade.
Touching the Brazil Cube, which can be summoned by an admin or RP Injur, can also grant you a free Dark Blade. Sometimes, defeating an admin in the game might reward you with the Dark Blade, although the chances are small.
Participating in a player raid against Me Hawk, a strong boss, gives you a low chance of obtaining the Dark Blade. Additionally, the Legendary Sword Dealer NPC sells the Dark Blade for in-game currency that you can earn through gameplay.
For an alternative, you can use apps like Playbite to earn Roblox gift cards by playing casual mobile games. These gift cards can then be used to purchase the Dark Blade in Blox Fruit.