Need help adding a C wire for a wifi thermostat

With the increasing popularity of wifi thermostats I know this question has been asked many times, however I’ll ask it again fr my specific situation.

How do I add a “C” wire to my existing air handler terminal and thermostat.

Here is a picture of the terminal (Magic Aire Dux-8 air handler)

Starting from the left, the C terminal has two wires a large White/Yellow and a green, neither of these wires come out of the wall where the thermostat is mounted. (The wire bundle that comes out wall has 6 wires – Red, Blue, White, Yellow, Green and Black. The Black is not in use at the terminal end or the thermostat end.) Next to the C terminal is the R terminal and has both the red and blue wires attached, part of the 6 wire bundle. The green wire is attached to the G terminal, also part of the bundle. Y has a yellow wire, part of the 6 wire group, and a red wire not a part of the wire group. W has a white wire, part of the bundle, and a large White/Blue wire not a part of the bundle. W1 is not in use.

Now for the thermostat image

This picture is self explanatory with two exceptions:
– the black wire mentioned earlier, part of the 6 wire bundle, is present but not in the picture.
– the blue wire is connected to Rc

So, now the questions are:
1.) how to I connect a wifi thermostat that requiers a C wire?
2.) can i simply use the unused black wire as a C?
3.) does the blue need to be connected the Rc on the wifi thermostat?