system
A chat on subreddit /r/NoStupidQuestions
human
They probably weren't always unmarked; they were just made of perishable materials that haven't survived 200 plus years (e.g. wood). (Before you get angry, this is in no way meant to say slavery/their treatment is any less shitty.)

As opposed to the sarcophagi of Washington and Mrs.

ETA: Also, there were many grave features, not just one unmarked grave where they threw them all.
gpt
Probably not very different from how they buried poor white people at the time.