Unless you know what it is, it's really not obvious at all. A flag at half-mast is a commonly known thing, someone important died. A 5 pixel black bar at the top of a site is not used anywhere but here.
Insularity, exclusion and gradually-learned shared secrets and customs are one of the ways communities define, distinguish, and stabilize themselves.
I recall black bars being used on various sites, although usually not for individuals. Similar to black mourning bands worn on clothing, or how sometimes newspapers will print black front pages. Black isn't a universal colour for death and mourning across cultures of course, but it is rather universal in western cultures at least.
At any rate, as I mentioned in another comment, I'm glad people ask when they don't know. And I'm not convinced having a link to an obituary wouldn't be a good thing, but apparently some people feel strongly against it.
You either know what it means or you don't. Did you know what a flag at half-mast meant the first time you saw it too? You learn what it means and move on.
Regardless of what you say it's not obvious... If it was there wouldn't be this thread.
I am aware of what the black bar represents however it is not always consistently used. for example, Ian Murdock who I consider to have a great impact on GNU/Linux in general was not given one after his passing.
The black bar is an admin/moderator decision, and because it's not applied to all front page deaths, it's hard for headline skimmers to associate the black bar with a consistent pattern.
That is why the black bar confused me - it's when the black bar isn't there that throws me off, because it makes me doubt the assumption that someone died.