Labelled on the wiring diagram for my blower is Cool (black, also labelled high), Heat (red, also labelled low), and M1 (blue, also labelled med).

The system is a gas furnace accompanied by an air conditioner.

Which one supplies power to the blower fan; black, red, or blue? My EXTREMELY limited knowledge of my HVAC system suggests to me that when the A/C is running, power is supplied by the black wire (high). When the gas heat is on, power is supplied by the red wire (low). True? Also, what purpose does M1 serve? I've read something about a park. Does this wire get hot even though it's quote-unquote, not-in-use?

Black, Red, and Blue are all the HOT feeds to the motor ( With white as the neutral leg)... depending on what fan speed ( motor speed ) you want, you connect the proper color lead to the heating pin on the fan board for the motor, which could be med. High, and generally High for AC due to the higher CFM air flow needed. The M1and M2 pins are dead pins used to park the un-used motor leads to so they are not just hanging around loose. There maybe some voltage to an unused motor leg from back electromotive energy generated by the motor windings.

Typically a motor will have a common, 2 capacitor leads, and 3 speed legs. The speed legs are simply wires connected to the armature at differing points to increase speed of rotation. The relay passes current through to 1 of the 2 (heat, a/c) and the 3rd (and others-some are 5 speed) is simply plugged into a "parking" terminal or taped off and tucked away.

The "park" terminal is simply a dummy terminal. It's not active, it's simply a place to park a wire out of the way and keep it from shorting against anything else. Quite a few manufacturers have parking terminals on their control boards.

So you will need to wire the Hi speed leg (probably black) to the Cooling or Hi terminal of the board and the low speed (probably red) to the Heating or Low terminal of the board. The white (and in some cases yellow) motor lead goes to the common wire/terminal, and the 2 cap leads (both brown) attach to the appropriate capacitor (see motor for required cap).

Check the wiring diagram on your furnace-it will tell you which speed goes where, and you can then check the motor diagram on the side of the motor and it will tell you which motor lead corresponds to which speed. Fairly simple once you know!

Good Luck, and I hope this helped.