LCD modules require some voltage,apart from power supply, for proper contrast. Some modules require +ve and some require -ve contrast voltage.

You will have to check from the datasheet what your particular module requires.Usually this voltage is applied at pin 3 of the connector.

Until you apply proper contrast voltage to your module, you wouldn't see anything on LCD. Black squares on the top line is an indication of proper level and polarity of contrast voltage.

I have some other experience with LCD modules. Sometimes there are jumpers on the PCB which have to be shorted by soldering in order for the LCD to operate. Again check the data sheet.