We need more info because the Blazer is available with 3 tire/wheel combinations. The LT models can have 18" or 20" wheel. The RS and Premier can have 20" or 21" wheels. What model Blazer do you have (LT, RS, Premiere)? Did you get an upgraded wheel/tire package? That info will help.
It would help to know what size you currently have. The size is on the sidewall of the tire. I have a 3LT with the standard wheels. My tires are 235/65-18 as indicated in the photo below. Look for something similar on your tires.