Yep - if the laptop display is black, but all is normal on the external monitor, you've confirmed a poorly-seated or defective LVDS cable or a defective laptop display.
You'll need to send 'er back to ASUS to get properly repaired. This time, PM cl-scott, the ASUS Customer Loyalty rep on this forum, with your RMA information, so he can help make sure your laptop is properly diagnosed.
G74SX-A1 - stock hardware - BIOS 202 - 2nd Monitor VISIO VF551XVT