AutoBluetooth
A tool that automatically enables and disables Bluetooth when you (dis)connect your Mac from external displays.
Using AutoBluetooth
Add AutoBluetooth to your Mac's Login Items (System Preferences : Users & Groups : Login Items), then log out and back in. If you disconnect all your external displays, AutoBluetooth will turn Bluetooth off. If you connect to an external display, AutoBluetooth will turn Bluetooth on.
Uninstalling AutoBluetooth
To disable AutoBluetooth, remove it from your Login Items and log out and back in.