This old Ask HN touches on the same question you are asking, as a black bar appeared on the Brussels event date and Andy Grove's death. I hope it clarifies that this black bar is not connected to an anniversary of an event.
For anyone else confused, today is the first anniversary of the Charlottesville incident, where violence occurred between the "Unite the right" group, and a counter protest. A car was also driven into protesters, killing 1.
Today a state of emergency was put in place by the state, to give specific state powers to the police, to keep the peace.
Has there ever been a black bar for a political thing or a terror attack or so? I don't think so.