The screen is completely black, but the power light is blue.
Your CFW image likely does not match your screen or motherboard revision. Re-verify your system info and flash an alternative version of the firmware. X12 Plus Custom Firmware
Locate the button to select the firmware file (often labeled as "choose" or "load"). The screen is completely black, but the power light is blue
Turn off the device and pop the MicroSD card back into your PC. The screen is completely black