Hi and thanks for your question. MIDI-over-bluetooth was included in MIDI Designer in Version 1.7.3 as seen here.
We use the built-in CoreMIDI-over-Bluetooth implementation by Apple. The implementation is opaque from our perspective (here's one starting point). Basically if you can connect from a Yosemite or better OSX machine, or an iOS 8 or better device, than your target is good to go.
We also support MIDI-over-bluetooth with certain devices including: