Just wanted to add that I’ve successfully paried a Bluetooth MIDI device on my custom Zynth with the onboard Bluetooth device using this guide (in case anyone else find it useful):
**Bluetooth service**
# Get the status of the Bluetooth service.
service bluetooth status
# If the Bluetooth service is not running, start it.
service bluetooth start
# Stop the Bluetooth service only if required.
service bluetooth stop
**Pairing a Bluetooth device**
# Once a device is paired it should automatically pair in future.
# Start the Bluetooth utility.
bluetoothctl
# Make sure the Bluetooth device is powered on.
power on
# Make sure an agent is running for the Bluetooth device.
agent on
# Start a scan for other Bluetooth devices in the area.
scan on
# Wait for the required Bluetooth device to be reported...
# Stop scanning when the required Bluetooth device is found.
scan off
# Attempt to pair the required Bluetooth device.
pair <dev>
e.g. <dev>=00:1D:A5:F7:FF:0D
# Pairing normally prompts for a password. Standard Bluetooth pairing passwords
# are: 0000 or 1234, try these if you are unsure of the password.
# If pairing fails or propt for password does not appear, try the following, and
# then try paring again.
agent off
power off
power on
agent on
# Once paired it should appear in the list of paired devices.
paired-devices
# You can now leave the Bluetooth utility and the device should be paired and
# ready for use.
quit
**Creating a serial device for use in the OBDII application**
# rfcomm associates the paired device ID with a serial device name.
rfcomm bind 0 <dev>
# The device it should create is:
/dev/rfcomm0
# To remove the serial device do the following if required.
rfcomm release <dev>
### Shouldn't need this command, force rfdevices to stop.
### rfkill list
**Unpairing a Bluetooth device**
# Start the Bluetooth utility.
bluetoothctl
# Unpair the Bluetooth device if required.
remove <dev>
# Make sure the agent is stopped for the Bluetooth device.
agent off
# Make sure the Bluetooth device is powered down.
power off
# Exit the Bluetooth utility.
quit
Source: Pairing Bluetooth Devices From A Raspberry Pi Command Prompt - Raspberry Pi Forums