You've got a couple routes you can go. Cruise control ultimately is a fairly simple device, typically using vacuum or speed sensors to maintain consistent speed.
Obviously the cleanest, most "stock" look is going to be original components, but they can be difficult to find and/or expensive if you can't pull from a junkyard. All the parts on the Blazer should be interchangeable, so it's just the matter of getting the right brackets, wiring, turn signal control, and components of the cruise control.
I know Ididit makes an add-on universal system that's pretty unobtrusive. It uses its own speed sensor (for non-computer applications) or can read pulses off the speed sensor (for computerized vehicles). I had a friend that used one on his muscle car and he said it worked alright.