The TV show is a spin-off on the 2 big-budget french films adapting Dumas' novel and being released in France next year (and yes we're talking about french-speaking tv shows here). The aforementioned "black musketeer" is a new character named "Hannibal", created for the films and played by french actor Ralph Amoussou, who most surely will reprise the role for the show. The character is inspired by an african prince named Aniaba who went to be a musketeer under the reign of Louis XIV (one century after the events of the novel and film so it's just an inspiration not a real adaptation of the real person). Not sure what role he has in the films (my wild guess is that he'll be more like an opponent to the 4 heroes and maybe like Rochefort befriend him in the end).

Remind me! 2 years