It's a platform using AT Proto (the protocol Bluesky uses), independent but still connected (think about Lemmy instances). That way they can apply their own moderation policies and don't have to follow Bluesky censorship.
More details here: https://blacksky.community/profile/did:plc:w4xbfzo7kqfes5zb7r6qv3rw/post/3lye7kyekw22m
There is another one coming up: https://northskysocial.com/posts/about