Old thread but I was wondering if you fixed your microwave?
Usually thermostats are closed (they let the current going through). This particular blue painted maybe be the fan control. It is normally open (circuit is broken) until it reaches a certain temperature (40C or so) then it closes and the fan kicks in. To test it, you have to warm it up a bit then use a voltmeter in Ohm position. It should read a resistance then.
Hope that helps.