A Black Moon is defined as the second new moon in the same month. That means that the sunlit side of the moon will be facing away from earth and the night sky will be completely dark! New moons aren't uncommon, but having two in one month hasn't happened in 2 years.
This will happen at 8:11 EDT Friday night. It will be a great night to stargaze, since skies will be nearly cloud-free and there will be no moon glare blocking the view.
It will be several days before we begin to see the sliver of the moon reappear.