Weapons: Claws, bare fists
The Black Belt is the upgrade to the Monk job with a few key differences. Once you unlock a Black Belt, however, there is no reason to keep a Monk in the party, as the Black Belt has notoriously high stat growth, especially in Vitality and Strength. In fact, one of the reasons you should invest in Black Belt and why it is so popular is because it is the only job that can almost guarantee your character will hit the coveted 9,999 HP cap at level 99. If you find your character lagging behind because their job has poorer stat growths, switch to Black Belt, grind them up a few levels, and you’ll notice an immediate change in their HP!
That aside, the Black Belt has the Boost ability where you store energy for a turn, then deal double damage next turn. It’s best to just attack each turn, rendering Boost (like the Monk’s Retaliate), fairly situational at best, and easy to ignore at worst. Be sure to equip your Claws when using the Black Belt job if you plan on investing in it, and you’ll probably want to job grind up to around level 40 as well. Unlike Monks, Black Belts need a lot of love before their bare fists out-damage claws.