As hardware, you need a BLE radio device, and a USB host (or OTG) device, and a microcontroller. (Both BLE and USB devices are typically integrated in a microcontroller, but I am not aware of any microcontroller that integrates both.)
Then you need to write software: a BLE-MIDI implementation, and host support for USB MIDI.
Teensy 3.6 if you want a USB host with full midi capability otherwise a teensy 3.2 or 3.5 will do and has midi over usb built in, then the Adafruit nRF8001 shield works great with the BlePeripheral library if you follow the SparkFun tutorial posted above you can easily create A midi peripheral.
I am not sure I am qualified to answer this but for what its worth my limited experience with using the audio (not midi) over bluetooth, bluetooth seems to introduce a lag and playing the instrument while listening on a bluetooth headset was not practical. Therefore I would imagine the same would go for MIDI over bluetooth. Since MIDI is real time critical you may be unhappy with the result so my gut feeling is that you may be wasting your time. Rod