However, you can expect to see figures in the range of 14-20 MPG combined (city and highway). The smaller engines might be closer to the higher end of that range, while larger V8 engines will likely be closer to the lower end, or even lower.
To get a more precise estimate, you should specify the engine size (e.g., 4.3L V6, 5.7L V8) and drivetrain (2WD or 4WD) of the particular 2000 Chevy Blazer you're interested in. Then you can consult resources like the EPA's FuelEconomy.gov website (though data for older vehicles may be limited) or online automotive resources.