Interesting coincidence... I also had a Blazer before the Av, and I also haven't had to turn on my headlights in something like 7 years. On the rare occasion that I drive the wife's car, I sometimes drive at night without the lights on because I forget that you have to actually turn the knob to get light!
While they are both controlled by the computer, and the light sensor plays into it, what you are thinking of are the automatic headlights. The DRLs and the auto headlights are basically opposites of each other. If the light sensor sees light, it turns on the DRLs and turns off the headlights. If the sensor sees dark, it does the opposite.
So, in theory, the DRLs should be on whenever the headlights are off, but there are a few additional requirements for the DRLs, as I stated above. The most common mistake, which bites a lot of people, is to put the transmission in park, hop out to check the lights, and they are not on.
Have a helper sit in the truck and keep the regular brake pedal pressed while shifting out of park. Then revel in the resulting amber glow.
-- SS