Stories and threads on HN are, for the most part, a sort of community property. The bar at the top of HN is not: it belongs to the operators of HN. Litigating how and when they use it to express grief is not an especially polite thing to do.

The easiest way, then, to understand the "black bar" is to think of it as an expression of sorrow that has a direct connection to HN's operators.