According to WGME, a "Black Moon" is the second new Moon of a month.

The last time it happened was three years ago.

A lunar cycle (from full moon to new moon and back) takes about 29 days to complete.  Since our months are a few days longer (28 to 31 days), every 32 months we get a bonus new moon or full moon.  It's the leap year of the lunar cycle!

The only downside is, as a new moon, you won't see anything.