Backlight Issue - Not Inverter, Not cable, Not LCD

Do you mean the white LCD display has video but no lightning when installed on the black MacBook (same for the black display) ? Maybe you damaged the logic board inverter circuitry on the black MacBook when doing your tests. There's a microfuse on the logic board that could be blown now so the inverter socket doesn't receive any power to feed the inverter board. The only way to be sure is to test the voltage inside the logic board inverter socket (or at the end of the inverter cable that connects to the inverter board). If the voltage is there then your problem is elsewhere if not then the logic board is faulty. A tech could make this verification.