Are there any Windows Phone 8 apps out there which can "remote control" your Windows 8 desktop or tablet PC via bluetooth, not via Wi-Fi? All I've found only works on the same Wi-Fi network or through a 3rd party webserver.
I don't think so, but that could be a possibility once Bluetooth connections gain the ability to be encrypted. At the moment, WiFi connections and 3rd party webservers are your best bet for remote control applications.
Check out Unified Remote. I use the Android version of it. It is $5 and you can use your phone as a remote for a number of programs and the Windows 8 start screen as well. I highly recommend it. It is one of my favorite apps. You should be able to use the app to Bluetooth from phone or tablet to PC. Use the trial version first before buying if you can.