a day when something very unpleasant or sad happens.
A bomb went off early this morning. This is a black day for the peace process.
(usually + for)
a black day
A day that marks a sad or unfortunate event. When we heard the news that David Bowie had died, we knew it was going to be a black day for all of his loyal fans.
a black ˈday (for somebody)
a day when something sad, unpleasant or disastrous happens (to somebody): It was a black day for this area when the local steel factory closed down.