If a thermostat was replaced did they flush the entire cooling system or just top off the Dex cool after it? Most Blazers start a lower intake gasket leak by 100k miles so look closely for that leak - is it in fact coming from one of the two WP holes or is it from the mating between the block and intake - a good light should be used. Then also look along the sides between the valve covers and intake and then feel the rear for leaks. Common is after a system flush or intake the Dex cool sludges up and takes the WP out (and heater core). Welcome to the GM 4.3 Blazer!