Why the display turns black & flashing during charging/cellphone is on

there are plenty of things that can cause this symptom. The basics is that it is either a software corruption, or the screen is recieving some of the charging current. this can be for multiple reasons. the process of solutions are as follows, from least-most expensive.

1) start with a factory reset, and see if it fixes the issue. (This will erase all of your data.)

2) re-flash the stock ROM to the device. (this will put new core files onto your phone. (a standard reset keeps the core files from your previous software. if there is a corruption in core files, you will not be able to get rid of it with a basic factory reset)

3) Charge port: There could be a short in your SubPBA, which will need to be replaced if that is the case.

4) Screen failure: There could be a short in your screen, causing a flicker when excess current is introduced to the device.

5) board level failure: a possible filter malfunction could be causing the issues. (to fix this you will need a multimeter, the Board Diagram -with indexes, and Surface mount soldering skills) if you do not have the required items and skills, then replacing the motherboard is also a solution.