Whilst performing a repair, there are minute issues that can impact a great lot on the functionality of the device. If the phone appears black but the phone still responds with sounds, then it’s most likely the screen itself was damaged during the repair. One option is to reopen the device and just check all connections and if possible clean the connectors on the logic board with a little alcohol and a q-tip but with a LOT of care. If these pins are damaged in any way, this can cause serious issues requiring further repair.
Once cleaned, redo the connectors and test whether the screen now works. If the problem still persists then please contact us through one of our various contact options.