I have an anusual problem , when I usb power my lcd screen along with an arudino uno everything work's like a charm , but when I try to put it on external power supply (9v 500mA) it just goes blank...
i hope you mean 9 volts on the arduino 7-12V DC input Jack and not on the display.
But maby the supply you are using right now cant actually handle it.
Try measuring the input voltage with a multimeter and try measuring the 5 and 3.3 volt rail
If it lower than when you are powering it by usb that means that the supply really cant handle it or maby there is something wrong with your arduino board