What does the Airspeed indicator show when your pitot tube is blocked?
Depends on how much blockage. If fully blocked, as in air tight, the pitot side becomes like the sealed aneroid side of an altimeter and it only reacts to changes in static pressure, indicating the difference between ambient static and the pressure trapped in the pitot side.  

If the blockage is not air tight, like you usually get with bugs, somewhere between zero and actual, in proportion to the blockage.  If the blockage is something that can move around, like a slug of water in the pitot line, you get wild and crazy fluctuations.