New Lcd is black but backlight on after digitizer and lcd replaced

Reseating ZIF connectors can be difficult, because you're not sure if you have gotten the ZIF connector to seat properly. You may want to look on the LCD again to see if there are any colored splotches on the screen or if the LCD end of the cable is working. While the LCD itself may not be easy to damage, the circuit boards in many displays are very fragile. I broke one just by touching it! If that doesn't fix it and you're at the point of eternal desperation, then you may want to get a new LCD and try that. If that doesn't fix the problem, you may also may want to inspect the logic board for any signs of corrosion. Hope this helps!

backlight no image usually means that the iPad is auto-booting into DFU mode. Connect to iTunes and see if it detects a device in recovery. Then try to update.