We can try to address this confusion.
When Pigsy asked, "But did anyone identify the body?", that was asked if anyone confirmed the body which the girl from Flowing Clouds screamed about previously in Chapter 377. Since he was stabbed almost immediately afterwards, no one did and there is probably no body/corpse. Because she was the one that said there was a body and also saying that it was probably Puppet Curse, the suspicion will fall on her as the body was unconfirmed.
As for what the Black man said about the nail. That is probably in reference to the spy he placed in the group being discovered.
We hope this cleared up any confusion.