Short answer is no. No car in the "G Body" family ever came with factory rear disc brakes, but I heard S-10 rear disc brakes bolt up without too much trouble. I'm just not sure if it's Blazer, S-!0 pickup, what years, or ??? If you HAVE to do junkyard stuff, do some research, otherwise Summit and Jeg's have rear disc conversions for not too much. I personally like new stuff, so if there's nothing wrong with your rear end, just go aftermarket and get a conversion setup...but that's my opinion.
**After your swap, if you're going to do this, look into an adjustable proportioning valve to dial a little pressure out of the rear brakes so they don't overpower the fronts mid-corner.