I am aware of an electrical engineer who figured out how to attach chip-level debugging gear to reprogram the circuit board so that it would again seek the USB for a failsafe load. Unfortunately, he was apparently contacted by so many non-technical folks wanting information or service(!) that he took down parts of his blog. I attempted the process with the help of an EE friend, but we were not successful.Have anybody unbrincked their Infotainment system after the black screen issue ?
I hardwired a serial port into the connector and tested with my working replacement unit, but the bricked unit never showed any activity, not even any bootloader failure messages.
My suggestion is to locate a CMU from any 2015+ (maybe 2014?) Mazda on eBay (~150USD), and swap its internal circuit board into your dead unit. I had to update the firmware from an ancient version to version 33 before I could update to 56, but the older board behaves identically after update.