Blazers have a bad habit of springing small leaks in the heater core. I've been smelling the same thing for a year. The core is a bear to get at - requires taking out the whole dash. I stopped by the Chevy dealer and got a set of six stop-leak tablets they sell for about $5. I'm going to give it a try and see if I can avoid having to pull the core.
Ugg, a mechanics nightmare
If your core is shot or bad, it pays to just take the weekend and replace it.
Try checking your hoses when they are under pressure, you might find a small pin hole leak. Sometimes the air from under the hood will add to that smell from the vents. If you don't find any, the there is a good chance its a pressure leak from your heater core.
They are a bear to get too, but you can do it without removing the WHOLE DASH. You just end up removing all the cover pannels for the lower part passenger side.