Block heaters heat up the engine coolant which in turn keeps your engine warmer than ambient. No, it's not going to keep it at operating temperature.
The block heater is operated by your standard wall outlet (i.e. 115 - 120 V AC in North America). You "plug-in" your vehicle in cold temperatures to keep the block warm. Plus, in extreme cold, you need to make sure that your water / coolant mix is good to -40 deg. C. I have seen cases where people have not plugged in their vehicles and the water / coolant mix is not good to -40 deg. C. Guess what happens? Water turns to Ice which expands and the block cracks.