I can't find any website for it. Are you sure it's not just some posting category on Bookface, YC's internal social network?
Might be this:
> Y Combinator is starting a blacklist for venture capitalists accused of sexual harassment: "'We don’t call it a blacklist, but that is essentially what is happening,” Kat Manalac, a partner at the influential start-up incubator Y Combinator, said of the blast email.
https://www.forbes.com/sites/lorenfeldman/2017/07/17/todays-...
Same. While it doesn't help that their name is about as generic as it gets, I searched across Kagi, Google, etc. and couldn't find any such YC company.
That being said, it wouldn't entirely surprise me if somebody's tried to start the tech equivalent of the casino "Black Book".
https://en.wikipedia.org/wiki/Black_Book_(gambling)