Many people don't think that the A/C is useful during winter, so many people get mad when you tell them that defrost uses the A/C- It uses it to remove the moisture in the vehicle (window fog), same as it does in summer (humidity), but doesn't run as long in winter due to cold temperatures. I also have an '01 blazer 2dr 4wd, which sets a code for secondary air injection pump (which I guarantee yours sets off). I do not understand what it is (lol), but I know the CEL flashes at speeds of 70mph+ w/ A/C (defroest) on. I have another code that keeps the check engine light on, so i only see it flash under your similar conditions, but my other code is a self check for the flapper valve for fueling the vehicle, so I can never get the tank 100% full; which BTW is caused by filling up while the engine stays running. I am listing this here, because I have hesitation with mine, which I have not scanned yet, but potentially might be the injectors (california there is a 200,000mile/10yr recall on those).