Finding "C" wire on old heat pump HVAC unit
The highlighted wire is NOT the C wire.  

If you read Note 3 it says "Transformer factory wired for 240 volts. Use red and blue leads for 208 volt applications".  The only time you'd use that blue wire, is if the system feeding the furnace was a 208 volt electrical system.  

If you look at the other side of the transformer in the schematic (labeled 24V), you'll see two yellow wires.  One is R, while the other is C.