Operate at 3.3V or 5V
According to the Atmega32U4 specification, the MCU can run at either 8MHz@3.3V or 16MHz@5V. Normally, you should set the voltage selection switch to 5V to run the board at 16MHz. However if you need to connect to shields or external components, such as sensors or MCU, which accept 3.3V only, you should change the operating voltage to 3.3V using the voltage selection switch.
Note that while Blend is running at 3.3V, the Atmega32U4 will still run at 16MHz which is outside Atmel's specification, the CPU is overclocked (we have not seen any issue so far).