When unarmed, each of the Black Belt's hands has a base damage of 1.5× the Black Belt's level, rounded up. When the Black Belt uses Boost, he becomes defenseless (physical Defense and Evasion effectively reduced to 0) and loses all Magic Defense until he attacks. If he uses Boost once before attacking, the attack deals double damage. If he uses Boost twice before attacking, the attack deals triple damage. If the Black Belt uses Boost a third time without attacking, an Overload occurs, Boost is negated, and the Black Belt loses half of his current HP.
Black Belts are excellent low-defense fighters once they become available, easily outclassing Monks and Scholars in that regard. Keep in mind that Black Belts will only attack with a single hand if they have a single weapon equipped, just like Monks. Make sure they are using either two or zero weapons.