Hello lfish, Starting in the year 1996, all GM vehicles are equiped with OBD2 compliant ECM's. When replacing the ecm in a 96 or newer GM vehicle, the new ecm has to be "flashed" or programed for the particular vehicle. Unfortunatly this is not a diy friendly thing. The vehicle has to be connected to a flash tool/scanner that only the dealers and a very few independent shops have. Without flashing the new ecm, the vehicle will not start.
What is the reason you replaced the ecm in the first place?
There are two things that must be done when a new PCM is installed.
(1) It must be programmed with the exact software calibration for your vehicle. This requires a scan tool with programming capability and a subscription to GM's corporate technical database to obtain the software to install. You state that the PCM has already come programmed; if the PCM is not programmed with the correct software what you will experience is that the vehicle will just crank and make no attempt to start. You have stated that the PCM has already been programmed before you installed it, so for the moment lets assume that it contains the correct software calibration. If the procedure I have provided below does not allow the vehicle to start, then there is a good possibility that the software included when you purchased the PCM may not be the correct calibration for your particular vehicle. If this is the case, you will need to have the vehicle towed to a GM dealer or an indepenent shop with programming capability to have it configured.
(2) Your Blazer has a factory security system called "passlock". The PCM is one of the modules involved in the passlock system, so when the PCM is replaced it must be programmed to recognize the other passlock components in the vehicle. I believe this is what the other person you talked to was trying to describe. using a GM scan tool, the passlock system can be programmed in about 5 minutes. If you do not have access to a GM factory scan tool, there is a manual programming sequence for the passlock system to allow the vehicle to relearn the security information. If the passlock system is what is preventing the vehicle from starting, it will crank over but there will be a "security" indicator lit or flashing on the instrument panel.
The manual passlock programming sequence takes at least 30 minutes to complete. Here is the procedure: