The Black Belt is a 7th quality tier item. It is dropped from Bone Lee, found in Hardmode Dungeons. It has a 7.14% chance to drop this item, as well as a 7.14% chance to drop the Tabi.
When equipped, the Black Belt has an 11.5% chance to negate any type of damage (lava, spikes, thorns, etc). Damage taken from Debuffs are an exception, such as On Fire!, Poison, and Cursed Flames. It will dodge the attack, but not any of the debuffs.
If the dodge activates while the player is wearing Turtle Armor, the enemy will still be damaged by the auto-thorn.