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.

You can do this perhaps using an Arduino DUE?
https://learn.sparkfun.com/tutorials/midi-ble-tutorial/create-a-basic-ble-peripheral

and
https://github.com/bbx10/USBH_MIDI/tree/due

Would require a bit of work though 🙂

Three versions in this article. None look like easy builds.

https://www.hackster.io/joebowbeer/usb-ble-wireless-midi-adapters-25dd31

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

It is easy to do with serial midi if available. With usb host it would probably be much harder.

I made a couple of adapters serial midi to BLE using an ESP32. Super easy, it only takes 4 wires a couple of line of codes with the midi pipe function of the "control surface" library.

The latency is minimal when I use it to play on my iPad, it's barely noticeable.

I posted the code on the control surface library github page if anyone is interested:

Easy DIN5 MIDI (Serial MIDI) to MIDI BLE on ESP32

A ESP32 / BLE example:

#include
#include

void setup()
{
pinMode(17, OUTPUT);
pinMode(16, INPUT);
pinMode(2, OUTPUT); // grün
pinMode(4, OUTPUT); // rot
Serial.begin(115200);
Serial2.begin(31250, SERIAL_8N1);
BLEMidiServer.begin("Basic Midi device");

Serial.println("setup");
}

bool blink = false;
unsigned long red = 0;
unsigned long green = 0;
unsigned long ms = 0;
bool connected = false;

void loop()
{
ms = millis();
//digitalWrite(4, !red);
if (ms >= green || ms + 100 < green)
green = 0;
digitalWrite(2, !green);
digitalWrite(4, BLEMidiServer.isConnected());

if (BLEMidiServer.isConnected() != connected)
{
if (connected)
Serial.println("wifi connection is broken");
else
Serial.println("wifi is connected");
connected = !connected;
/*
BLEMidiServer.noteOn(0, 69, 127);
delay(1000);
BLEMidiServer.noteOff(0, 69, 64);
delay(1000);
*/
}

while (Serial2.available())
{
// im 250 ms Takt wird 0xFE gelesen (active sensing)
char ch = Serial2.read();
Serial2.print(ch);
//Serial.println(ch, HEX);
green = millis() + 2;
}
}

check this video, maybe it will help u)

You can build one with a Raspberry Pi Pico W and a few connectors. Project details here: https://github.com/rppicomidi/ble-midi2usbhost