From Bulbapedia, the community-driven Pokémon encyclopedia.

A Black Belt (Japanese: からておう Karate King) is a type of Pokémon trainer that first debuted in the Generation I games. They are generally depicted as young men in headband and gi striking a fighting pose.

They specialize in Fighting-type Pokémon.

Their female counterpart seems to be the Battle Girl or Crush Girl.

Appearance

Black Belt Sprites

Image from Pokémon Red and Blue

Image from Pokémon Gold and Silver

Image from Pokémon Ruby and Sapphire

Image from Pokémon FireRed and LeafGreen

Image from Pokémon Diamond and Pearl

Generation I-introduced trainer classes
Beauty | Biker | Bird Keeper | Black Belt | Boss | Bug Catcher | Burglar | Champion | Channeler | Cooltrainer Cue Ball | Elite Four | Engineer | Fisherman | Gambler | Gentleman | Hiker | Jr. Trainer♂ | Jr. Trainer♀ | Juggler Lass | Leader | PokéManiac | Trainer | Psychic | Rival | Rocker | Rocket | Sailor | Scientist Super Nerd | Swimmer | Tamer | Youngster
Beta classes
Chief | Pokémon Prof.