My wife bought a Hunter programmable thermostat to
Normally the common wire is black, but installers can use anything, so you need to make sure that blue came off a "B" labeled screw. The common would normally be labeled "C". You need a simple multi-meter to check, although I have to believe that the instructions with the new thermo would have to cover the hookup and should show some extra connections if truly universal, as it would have to accommodate heat pumps and dual stage systems. With the old thermo mounted on the wall as it was originally with no wires connected, check for continuity between R and each of the other screws. Set thermo for cooling and turn temp down to 60 and see if you get continuity R to Y, set fan to on and see if you get continuity R to G, set thermo to heat and see if you get continuity R to W. If you just have a single wire that runs to the contactor in your condensing unit, and one wire for the heat, then the red is power from the transformer in the furnace, green is for the fan on all the time during cooling, yellow for the AC contactor, and white to bring on the heating cycle, IF ALL are single stage (only one setting for heat, cool, or fan that is either on/off). Check for continuity between R and B on each possible setting. If you get no continuity or greater resistance than on the other screws (when checking R to B), set meter to VAC and see if you get 24 volts between red and blue wires - the wires coming out of the wall and not the thermo screws. If so then you have a simple single stage for both heat and cool system. If you tape off the blue wire so that it can't contact anything in the new thermo then hook up the other wires as you state and see if everything works as it should. The blue wire in that case is a common that is not needed in your new thermo as it has its own internal battery power and does not need the common to power it from the transformer. Some systems actually use 2 transformers and the common does not run to the thermo as the other leg of each circuit is terminated elsewhere, but another wire does. If so, you will not get the readings above and you may need to do some rewiring - this is very unlikely.
