Thermostat C Wire (Possibly Unconnected)
You need to open the air handler and confirm the wiring. Never trust the color of the wires. If I had to guess in a hurry in a panic situation I would try the blue connection. But you are right in front of it. You are two screws away from confirmation.

Shut the power off at the switch first.


  How to tell if your B wire is really a common wire in disguise:
  
  Read through the manual for your old thermostat.
  If you have the manual for your thermostat (or can find it online), it may describe the function associated with the B connector and help you determine if it’s meant for a heat pump wire or a common wire.
  
  Does your thermostat have a connector labeled C?
  Whether or not there is a wire attached to it, if your thermostat has a separate connector labeled C in addition to a connector labeled B, then the wire labeled B should be connected to the O/B connector when installing the Nest Learning Thermostat or to the /OB connector when installing the Nest Thermostat E.
  
  Do you have a heat pump?
  It helps to know what kind of system you have. If you have a heat pump and you don’t have a separate wire labeled O, then the wire labeled B should be connected to the Nest Learning Thermostat's O/B connector or to the Nest Thermostat E’s /OB connector.
  
  What color is the wire labeled B?
  While the color of the wires connected to a thermostat often don’t adhere to industry standards, they can provide a clue in situations where you’re still uncertain about the function of a wire labeled B. If your thermostat doesn’t have a separate connector labeled C, and you’re not sure what type of system you have, make a note of the wire color connected to B. A brown wire is mostly likely a heat pump wire and should be connected to the O/B connector on the Nest Learning Thermostat or the /OB connector on the Nest Thermostat E. But if the B wire is blue, it should most likely be connected to the C connector.


That's from Nests website

I don't work with heat pumps enough to say "yes connect to B"

I would look at how the system is wired up and determine if it's suitable as a common.

There is one terminal in the image that is not visible, is it a C terminal?

EDIT

To conclude, 

This is as far as I know, unique to Trane heat pumps. Trane only employs the O terminal for the reverseing valve and do not mark or identify a C terminal and substitute it with the B terminal. Seems odd to me but Trane does do some weird things. 

Anyway you should be good to go on the air handler B terminal.