4 Possible answers to this:

1. The digitizer is bad - this is uncommon but not unheard of. Roughly %5 of the screens my shop gets in are bad. Most distributors accept returns on defective parts.

2. The digitizer isn’t plugged in to the board

3. The digitizer port on the motherboard is damaged - this can be tested by trying a known good screen.

4. Software issues. This is most common in iPhones, in my experience. So far as I understand, parts can have hardware identifiers similar to a MAC address. Suddenly changing the hardware can “confuse” the software. You can try a force reset or resetting the phone.

You’ll need a LCD/Digitizer assembly such as https://www.amazon.com/Screen-Digitizer-.... The black marks are almost definitely breaks in the LCD screen.