A Black Moon is when you have 2 new moons in one month. It is opposite to the more well known, Blue Moon when there are 2 full moons in one month.
According to NASA, "Black Moon" isn't even a scientific term. It also has been used to describe a month that doesn't have a full moon, or to describe a third full moon in a season that has four full moons.
By the way...there will not be a Blue Moon in 2017. The next one will be on January 31, 2018.