Test voltage for common on HVAC wiring
The blue wire is unconnected (floating) at the moment

I can see from your photograph of the wiring at the air handler that the blue wire in the thermostat cable is simply disconnected at that end.

Finding your C wire (using the air conditioner trick)

See how the yellow wire from the thermostat is wire nutted to a red wire going off into a second brown cable? That second brown cable is connected to your air conditioner's outdoor unit, and the white wire in that cable must be connected to your C wire in order for you to have air conditioning.

So, nutting the blue wire from your thermostat-cable into where the white wire from the air-conditioner cable is joined into (after stripping the end of the blue wire of course) will make your blue wire the C wire you're looking for.