is that an exhaust wastegate as a blow off valve....?

No, it's a wastegate thats being used as a vacuum operated supercharger bypass valve. See the setup below.

Attachment 4683

At idle, the wastegate is open allowing air to bypass the charger and maintain a steady idle. Depending on the spring rate, the wastegate slowly shuts the more you put your foot down on the accelerator until the charger engages and holds the wastegate shut.

The wastegate was used for 2 purposes.

1. Maintain a consistent smooth idle by bypassing air around the S/C

2. To help smooth/cushion the violent transition into boost.

Attachment 4682

You can see from the dyno sheet that between 3200rpm and 3300rpm there is a violent increase in torque and power. This all happens in a very short period of time. The wastegate helps take the 'shock' factor out.

http://www.diysupercharging.com/