They're brown\tan Japanese people. Like how they are in many anime there is a token darker skinned person for design diversity but the character is still suppose to be Japanese.
Even Matt, he's just heavily baraface.
I think Lenora, Marshall, Iris, and Grant are the closest thing to black people in Pokémon, and even then we don't know if their skin is like that due to IRL-type racial genetics or it is a fantasy-like feature like how many Poképeople have green or blue or pink hair.
