How can I install a Honeywell RTH65800WF Wifi thermostat?
I've labeled your photo, which should help you understand what's going on.



What you describe as a "blue" wire at the thermostat, you describe as "gray" at the air handler. I can't be sure, since I'm not there, but I think they are one in the same.

If you want to install a fancy new WiFi thermostat, you're going to have to pull an additional wire from the air handler to the thermostat. Once you have that, you'll connect one end to the C terminal of the air handler, and the other end to the C terminal of the thermostat.