Anime has come a long way from its original non-diverse era. Nowadays, there have been more and more Black characters being introduced, even as protagonists. It is a sign of the art evolving, but also the addition of creators like LeSean Thomas, Takashi Okazaki, Shinichirō Watanabe, and Aaron McGruder, among others, who have made an effort to include more Black characters and their stories.
This was a way for animation studios to start straying away from Black caricatures but felt that Black characters weren't marketable yet. Piccolo has grown to be one of the franchise's most beloved characters and has seen an arc matched only by Goku and Vegeta.
This is never really touched on in the series, but their depictions seem to be that they could be bi-racial. Whatever the case, these two characters are essential to the story of One Piece, being that Usopp is one the mighty pirates from the Straw Hat Crew and Kuzan, the former marine turned Black Beard pirate, has been one of the more formidable antagonists that Luffy and the Straw Hat Crew have faced.