Googling revealed this: "When someone important to the community dies, a thin black bar is added to the top of HN as a mark of respect."[0]

[0]: https://news.ycombinator.com/item?id=5990015

This is what the black bar looks like:

http://web.archive.org/web/20111006185350/http://news.ycombi...

On HN the top orange[1] bar is sometimes changed to black when someone significant[2] to computing dies.

[1] orange unless you've got enough karma to set it to some other value

[2] significant to computing is not the right phrase here, sorry for clumsiness.

Perhaps just: "someone significant to the community".

got it, thanks for the explanation. I figured it was something like that, but wanted to be sure :)

Usually, it means adding a black bar to your website header with a message commemorating someone who has passed.

I'm guessing that the default orange header bar would be black.