this rim/tire size would fit my 03' blazer?

18-inch (460 mm) forged, split-spoke wheels with P225/40R18 tires

they're the stock rims off the 08' cobalt ss

 

No. Cobalt bolt pattern is 5x110 and the Blazer is 5x4.75 or 5x165something or other in metric. Besides, 225 is a skinny tire for the Blazer. Stock is 235 and most people usually put wider tires on if they go aftermarket.

Also it would help if you the platform of your vehicle in your signature. I see the 03 Blazer but is it 2wd or 4wd?