Some progress on this - it appears to be bad drivers supplied by RedBear.
To program the BlendMicro you must install the board’s driver as instructed on RedBear’s code repository here: GitHub - RedBearLab/Blend: Arduino-compatible boards (Blend and Blend Micro) with BLE (Atmega32u4 + nRF8001).. The ‘RBL_Windows_Driver_Installer.zip’ driver package needs to be installed as instructed by RedBear so the board can be detected in the Arduino IDE HOWEVER the driver also completely removes the HID Keyboard element of the board.
After programming the BlendMicro on a computer with the required driver installed, if I then remove the BlendMicro and connect it to a different computer WITHOUT the driver installed - it detects it as a HID Keyboard and it works fine!
It appears some work needs to be done on RedBear’s driver for this board…