BlueDevil is the KDE Bluetooth stack. It sits on top of BlueZ to provide functionality that's well-integrated with the KDE Plasma desktop. This page mainly covers BlueDevil-specific functionality. You may need to refer to the BluetoothUser page for general information on Bluetooth in Debian, or the BluetoothUser/a2dp page for information on Bluetooth audio in Debian.
Install the bluedevil package to obtain the KDE Control Module, KIO integration, device pairing wizard, tray applet, and daemon for listening for new pairing requests. This will also pull in bluez if it hasn't already been installed.
When connecting a Bluetooth dongle to your PC, or if your hardware has built-in support for it, a Bluetooth applet should appear in your system tray. When opened, it will prompt you to add a new device via the BlueDevil pairing wizard. After a device has been successfully paired, it will be listed under an "Available devices" header inside the applet's menu, for quick connection and disconnection.
You can access the full Bluetooth settings by either finding the "Bluetooth" section in KDE System Settings, or clicking the "Configure Bluetooth..." button in the top-right of the tray applet.
From the settings panel, you can click the "Configure..." button in the bottom-right to change your computer's advertised name, view your address, toggle visibility, set your policy for file transfers, as well as set your folder for receiving remote files.
With a compatible device connected, you can right-click on files in Dolphin, hover over "Share...", then click "Send via Bluetooth" to transfer that file to your device.
There are some intricacies with adding Bluetooth audio devices (microphones, headphones, headsets, etc.), and if you encounter an error when trying to add one, you likely need to first refer to the BluetoothUser/a2dp page.