This is a problem with your door switch.
Its actually a common problem, and its why the blazers have that stupid 'dome light override' switch... come on, seriously, GM you need to fix your vehicles, not just mask the problems.
Pull each door switch out in turn with all other doors closed, and wiggle them around

Eventually when you find the right one, the problem will go away.
Also, there should be some codes on the computer as well if you have a theft lock II (it reports the codes back for whatever reason). Strange I know, but its what happens. Rain / washing the car, can cause it, or it can just happen on its own as well.