As the furnace wiring diagram shows, it is the "neutralish" side of the secondary of the 24 VAC transformer. It is marked "brown" on the diagram and goes to a round doohickey (probably a small motor).

Many HVAC technicians aren't so bright so to avoid future hassle, mark the blue wire you commandeer as C or color it brown with a brown marker pen.

As the furnace wiring diagram shows, it is the "neutralish" side of the secondary of the 24 VAC transformer. It is marked "brown" on the diagram and goes to a round doohickey (probably a small motor).

Many HVAC technicians aren't very flexible on wire colors so to avoid future hassle, mark the blue wire you commandeer as C or color it brown with a brown marker pen.