For those who don't know, a black bar is for people that die that HN honors by changing the `topcolor` color to black. You can do it yourself in your profile.
Fun history trivia: A few years ago the black bar used to be on top of the topbar. [0,1] Now it looks like this [2]
[edit] I can't find any very recent deaths that confirm the change so I may be wrong. Whoops.