Precise location can vary slightly depending on the engine (4.3L V6 or 5.7L V8, etc.), but it's generally in that area.
What it looks like:
It's typically a small, cylindrical or slightly rectangular metal valve. It might have one or two vacuum lines connected to it (for vacuum-controlled systems) and two coolant hoses attached. It might also have a small electrical connector (for electronically-controlled systems, less common on this model year). It often has a noticeable cable or rod connected to it if it is cable-actuated, though many are vacuum operated.
To be certain, you'll likely need to consult a repair manual specific to your Blazer's year, engine, and trim level. A visual inspection of your vehicle in the area described above, comparing what you see to images of heater control valves online (search "2000 Chevy Blazer heater control valve" on Google Images) should help confirm the identification. Remember to allow the engine to cool completely before attempting any inspection under the vehicle.
