welcome to AF.
i guess the most important thing to look for on a blazer or jimmy would be regular maintainence. these trucks like to go through things like front end components (ball joints, tie rods, etc), brakes, and u joints (just to name a few) so the more things the previous owners have done the better. most blazers i see have between 70,000 and 200,000 miles on them. its sort of a toss up wheither you buy one with lower miles and things my come up or to buy one with high miles where most regular maintanence items have been replaced.
it also depends what year and price range you are looking in. Gen 1 s-10blazers (82-94) are the more squared off style and will have engines ranging from 2.8-4.3. on the 92-95 blazers with CPI (intake manifold with 6 runners and vortec plate) watch for leaking injectors and fuel lines (nut kit). signs of this would be sluggish performance, and a bad gass smell coming from the exhaust. these blazers can be had for a fairly low price.
The newer Gen 2 blazers (95-05) will have more "bells and whistles", which will make them more expensive then gen 1's. in 96 the engines switched over to the newer CSFI units which increased fuel economy. these do eventually start to leak over time (like CPI units). also the lower intake manifold gaskets will also start to leak. very important, on these gen 2 blazers replace the factory rad cap as they are defective. stant is a good replacement.
overall, blazers are good trucks. they do take on a personality of there own though
. if you have any other questions just let us know and someone will be happy to help you.