2004 Chevy Blazer

A drain on your battery when the vehicle is off is also known as a parasitic draw. To diagnose this accurately you will need a multi- meter with an amp probe. The most likely culprits I have seen are door jamb or headlight switches, something that does not require the ignition to be turned on in order to get power from the battery. I would first check to make sure all your interior lights are working - especially the dome light and step/courtesy lights in the doors if you have them. If there is ANY electrical issue you can think of, even if it seems unrelated, I would look there first.