My Mac mini is a late-2012 model (Macmini6,1).
It has a problem with the shielding around the Bluetooth module which makes it very susceptible to RF interference from the nearby USB 3.0 ports, cables, and so on. It's a known problem and one of the better known sites for Macs even sells a new shielding kit. Without the shielding upgrade, I've had nothing but problems with Bluetooth peripherals. The only BT peripheral I've ever been able to use is an Apple Magic Mouse 1.0.
I'm a little paranoid about pulling apart my Mac mini to install a shielding kit, so I bought the Laird BT820 dongle.
I plugged the BT820 dongle into a USB 2.0 mini-hub plugged into a USB 3.0 hub. Strangely, I was able to pair the TextBlade right to the dongle (as well as an Apple Magic Mouse 1.0) without having to take any other action other than plugging in the dongle and pairing the TextBlade (and mouse).
This made me curious, so I downloaded the Hardware_IO_Tools_for_Xcode from Apple's developer site (as described on the Wiki and ran the Bluetooth Explorer. What appears to happen is that when plugging in the Laird BT820 dongle, OS X (10.11.6 (15G31)) is automatically disabling the internal Bluetooth hardware.
The configuration persists across OS restarts; I've also unplugged and replugged the BT820 multiple times and the internal Bluetooth is always disabled and the BT820 becomes the active Bluetooth support hardware.