The blueNES 2 is a follow-up to the blueNES. Instead of using an Arduino, the blueNES 2 uses Adafruit's Bluefruit. When connected to ground pins 0 - 11 on the Bluefruit will send a keyboard command.
I've connected mine to an old NES controller. It's that simple.
The controller pinout I found on this Instructables.
Check out the build photos and if you have any
questions let me know on Google Plus.