This worked for me; it might not work for you. Remember I’m just same random guy on the internet so I take no responsibility for anything that happens to your machine. Okay?
As yet (5th Aug 2015) there’s no official support from Apple for Windows 10 running through Boot Camp. So, when I upgraded from Windows 8.1 to 10, I was expecting there to be a couple of issues. One of these was a lack of Bluetooth. Also getting the fn key working.
But, it turns out to be easy to fix.
Get the Driver
The driver for Windows 8.1 works with 10, so you just need to grab that.
Go to https://support.apple.com/en-us/HT204048 and find your machine in the list. You want to look for the zip file download of the Boot Camp Assistant software.
For me, it was this one:
Download that file (it’s pretty big – 882Mb for my one) and unzip that once done.
Install the Driver
Inside the unpacked folder, go into the BootCamp folder and then into the $WinPEDriver$ folder under that.
The driver needs to be installed via device manager so just confirm that the AppleBluetoothBroadcom64 folder is there for now.
In Windows, open up device manager (the easiest way to do this is just hit the Windows key and type ‘device manager’ – it’ll be the first result).
Expand other devices and you should see an unknown device.
Right click it and select Update driver software.
Chose the second option on the pop up to browse your computer for the driver software.
Browse to the $WinPEDriver$ from your downloads and select that. Make sure Include subfolders is checked and then click Next and Windows will install the Bluetooth driver for your Mac Book.