Wiring – Should I leave splices when pulling new thermostat wire

Almost every thermostat installation manual I've read always says something along the lines of "Make sure to mark you wires based on the currently connected thermostat. Do not rely on the wire coloring".

The colors are really just a guideline, there is no requirement to adhere to them.

Looking at the controller board of your furnace should confirm how they are connected. It's possible that the blue wire isn't even connected to anything on the furnace side. Perhaps the furnace was replaced at some point and the thermostat wasn't re-wired.

It's not uncommon to not have a C wire as older thermostats were either unpowered or powered by batteries. New thermostats (especially the ones with LCD screens, Wifi, etc.) have larger power requirements and rely on the C wire to provide a return path for power coming from the R terminal.

Ultimately I figured out what I was doing wrong with help of this question: Adding a C Wire To A New Honeywell Wifi Thermostat

I was mistaking the 24VAC transformer for the relay to the outdoor unit. The only thing I really needed to do to the original wiring was connect the blue wire ("C") going up to the t-stat to the splice bundle near the furnace.