Thermostat C-wire not working

I recently switched out my old thermostat for a new thermostat. The previous thermostat had an R, Y, W, G, and C wire. I plugged all of these wires into the same ports on my new thermostat.

After doing this, the new thermostat does not detect power on the common wire and the blower stays on 24/7. I even removed the G-wire from the thermostat and the blower still comes on.

The thermostat i’m using is a Nest which does not require a common wire. I removed the common wire and now everything seems to be working. The blower, AC, and Heat all work and only come on when called.

I guess I can leave things as-is, but I’d like to know why my ‘common’ wire did not work. I went down to the furnace to try to match the wires by color, and the common wire connected to the furnace board is black even though the wire up by the thermostat is white. What’s the best way to determine what this wire is and whether I actually have a common wire?

Thanks in advance for any help!