HVAC wiring for wifi thermostat installation (Ecobee) gas furnace + AC
It is normal for a 24vac control transformer to measure somewhat high under no load (i.e. 27v vs 24v).  It is 24vac system.

"B/C" on the furnace should be the common.  I am surprised the existing thermostat works connected like it is.  The only thing I can think of is that the thermostat is internally configured for a single power supply system and the "Rc" terminal is open instead of being shunted to Rh as is the case with many thermostats.  In this case if Rc were shunted to Rh it would pop the overload protection on the furnace board or the transformer.

Should be good to go on connecting the black wire to C on your new thermostat.  Do not connect any of the wires to Rc on the new thermostat.

Edit:

Schematic for Goodman AMV9/ACV9_XA furnace: