yeah you're in Zona Sul, you're not gonna be around a lotta black people compared to other places, but it's not like Leblon where you'd think they have security at the borders of the bairro to turn darker people away
the thing about beaches, especially in zona sul, is that people come from all over the city to chill there. so you got a crew of

right next to the brazilian GBE
a lot of my black brazilian friends (and by that i mean those that actually consider themselves black brazilian, because i know a lot who would be considered black in the US, but not in Brazil) live in Jacarepaguá, but that's a mixed area. if you got heart it's near Cidade de Deus if you wanna be around "o povo". you could also go to Rocinha, which is a favela but has crazy development, full of afro brazilians and hood rich-osity.
be sure to hit up Lapa at night time
#1 thing is be careful tho. not paranoid, but careful. dont be stupid and think it's all good all the time, because regardless of skin color, many will consider you gringo and assume you're wealthier than they are, and they're probably right. you going solo?