It has also occasionally been applied to at least four different situations:
1. The second occurrence of a new moon in a calendar month.
Notes - Cannot occur in February. Analogous to the common calendrical definition of a blue moon for months with two full moons.
2. The third new moon in a season that has four of them.
Notes - Analogous to the Farmers' Almanac definition of a blue moon for seasons with four full moons.
3. The absence of a full moon in a calendar month.
Notes - Can only occur in February, thus January and March will each have a second full moon (a calendrical blue moon).
4. The absence of a new moon in a calendar month.
Notes - Can only occur in February, thus January and March will each have a second new moon (see definition 1).
Picture courtesy of Elemental Magick Magazine
Text by http://en.wikipedia.org/wiki/Black_moon