I purchased a used 96 blazer with just over 72,000 miles on it about a year and a half ago. Everything was fine until I hit the 97,000 mile mark, then the water pump went out, the fuel pump went out (spent over $600 finding and repairing that problem), and the transmission sounds like a tie fighter from Star Wars. An old lady had owned it previously and keep very detailed records of every oil change, repair, ect... All I'm saying is BE CAREFULL! Typically, blazers have transmission problems and it appears that the fuel system has problems also. However, everyone tells me that the 4.3L engine is good and will last. The question is will everything else around it.
Finally, I bought it cause it was cheap and appeared to be well taken care of, but If I had it to do over again, I would probably have saved-up and purchased a japanese vehicle. If the Blazer your looking at has high miles, maybe the owner has already replaced everything and your problems could be minimal. Good Luck!