Dang, pg or whomever tweaks the CSS on days like this, while today's events weren't specific to our community they affect us all. Flags are being flown at half mast, let's pay our respects.
I think HN hasn't done this in the past for attacks, e.g. there was no black bar for the Paris terrorist attacks, or the Ukrainian airliner shootdown. Mostly has just been a way of marking obituaries of specific individuals.
If we had to put up the black bar every time there was a mass shooting or murder in the US, it would never come down.