Currently Bluetooth support is limited to AVR based chips. For Bluetooth 2.1, QMK has support for RN-42 modules and the Bluefruit EZ-Key, the latter of which is not produced anymore. For more recent BLE protocols, currently only the Adafruit Bluefruit SPI Friend is directly supported. BLE is needed to connect to iOS devices. Note iOS does not support mouse input.
Not Supported Yet but possible:
HC-05 boards flashed with RN-42 firmware. They apparently both use the CSR BC417 Chip. Flashing it with RN-42 firmware gives it HID capability.
Sparkfun Bluetooth Mate
HM-13 based boards
Currently The only bluetooth chipset supported by QMK is the Adafruit Bluefruit SPI Friend. It's a Nordic nRF5182 based chip running Adafruit's custom firmware. Data is transmitted via Adafruit's SDEP over Hardware SPI. The Feather 32u4 Bluefruit LE is supported as it's an AVR mcu connected via SPI to the Nordic BLE chip with Adafruit firmware. If Building a custom board with the SPI friend it would be easiest to just use the pin selection that the 32u4 feather uses but you can change the pins in the config.h options with the following defines:
define AdafruitBleResetPin D4
define AdafruitBleCSPin B4
define AdafruitBleIRQPin E6
A Bluefruit UART friend can be converted to an SPI friend, however this requires some reflashing and soldering directly to the MDBT40 chip.
This requires some hardware changes, but can be enabled via the Makefile. The firmware will still output characters via USB, so be aware of this when charging via a computer. It would make sense to have a switch on the Bluefruit to turn it off at will.
Use only one of these
BLUETOOTH_ENABLE = yes (Legacy Option)
BLUETOOTH = RN42
BLUETOOTH = AdafruitEZKey
BLUETOOTH = AdafruitBLE
This is used when multiple keyboard outputs can be selected. Currently this only allows for switching between USB and Bluetooth on keyboards that support both.
Automatically switch between USB and Bluetooth