Black Iron Oxide, also known as Ferrous Oxide (FeO), is red iron oxide that has been fired in a reduction atmosphere. Iron oxides are the most common colorants used in the ceramics industry. Black Iron Oxide specifically can be used as an alkaline flux in clay bodies and glaze recipes.

In glaze:

1% of black iron oxide will give you a pale green 

3% of black iron oxide will give a greenish gray color

8% will give you a black

Information gathered from The Potter's Dictionary of Materials and Techniques by Frank Hamer.