In the Extreme NXT book we show you how to make your own Light sensor using a CdS photocell or Light Dependant Resistor. You don't need a special NXT-G block because the LEGO Touch sensor block has an output that is the RAW value from the A to D converter. However, unlike the LEGO Light sensor, you can't control an LED that could be a useful light source for reflective or transmissive sensing.
Below is the schematic of a CdS based Light sensor with an active LED control. The state of Pin 5 or the yellow wire in the NXT cable can be controlled in software by selecting the sensor type as LIGHT_ACTIVE or LIGHT_INACTIVE. The LabView sub VI for the CdS Light sensor block is also illustrated below.
CdS Light Sensor with LED Control
The NXT-G block and its configuration window are shown below. To keep things simple, the block only outputs the RAW value. It will range from 0 to 1023 with lower numbers for brighter light.
Here is a simple NXT-G program using the sensor block. The state of the LED is shown on the block with the little sun like icon in the lower right just like the real LEGO Light sensor.