I have a Lenovo G50-30 and i recently received the LCD cable as well as a new Screen and replaced both. The screen still appears black but it works perfectly once i plug it into a external monitor. What could there possibly be left to do at this point ?
Try to enter the BIOS to see if you can see anything on the display. Doing that removes any possibility of a software issue.
Also try to shine a flashlight on the screen and see if you can detect faint display. If so, the backlight may be defective. The backlight is part of the screen assembly is most cases but there is hardware on the motherboard that drives that. That may be broken. The same is true of the drivers for the display.
Assuming you connected the cable properly, there may still be a HW problem with the motherboard. Could be either the circuitry driving the backlight or driving the display.