The annoying 'What's the Black Line For' Treasure Hunt every time it happens could be eliminated by having a tooltip appear on hovering over the black line with something along the lines of "Bill Smith dies."

For extra points, a <LINK to some News Article Telling Us about Bill Smith> could be included.

If Google can do that when it shows a 'special day' image, why can't HN??

it's an indicator of a death within the community; an 'in memorial' feature.