The 2001 Chevy Blazer does have an ignition security system that is controlled by the computer in the vehicle. The light might come on because of an error that is coming up in the computer due to a key problem or an ignition problem. This is a common issue with the 2000 and 2001 Blazer.
The Security light "on" means that your VATS, or vehicle anti theft system is in operation. therefore, to deter theft, it won't start. Usually, putting the key in the door lock and turning it will do the trick.