“Legendary Black Belt” inflicts damage based on the original DEF of the destroyed monster (in the Graveyard).
If a monster equipped with “Legendary Black Belt” is destroyed, no damage is inflicted by the effect of “Legendary Black Belt” because it is no longer on the field. This includes if it attacked a monster with equal ATK, for example.
After the monster is sent to the Graveyard after being destroyed as a result of battle, the effect of “Legendary Black Belt” that inflicts effect damage can be chained to.