Black boxes are what are displayed if the display is never initialized. That means you've got a bad connection to the LCD generally, or something bad has happened to the LCD and it is now non-functional. You might try touching up the solder connections on the pinheader and verifying that none are shorting.
the solder on the 12 pins connecting the display board to the main board look fine, but I will touch them with a soldering iron just to make sure.
Digikey has the display assembly corresponding to the part number on the sticker on the back of the display board (different from the part number in the 4.2 hardware BOM) for $15. this will put my desoldering skills to the test. the part number on the sticker is NHD-0216K1Z-FSW-FBW-L.