The Black Cat mezzanine board extends the PEXARIA5 board with I/Os. However, the WR1 connector on Black Cat is no longer used, as the White Rabbit circuitry is implemented on the carrier board.
Originally, the Black Cat Board has been designed as a piggy module for the PEXARIA2A PCIe card. It has been designed by Jan Hoffmann / EE. It's purpose is twofold. First, simple I/O should be added to the PEXARIA2A card. Second, PEXARIA2A should be extended by White Rabbit capability. Today (10/2013), Black Cat is no longer used on PEXARIA2.