Lurk Mechanic

Lurk {1}{b} ({1}{b}: Turn this card face down. While face down it is a 3/3 creature with no color and no creature type. Turn it face up at any time by discarding a card.)
3/3 creature has a base cost of {g}{g}, but since this is black it is {b}+{1}{b} to get your 3/3. Of course these cards should do something when they flip face up, and that will increase the cost accordingly.