'69 was definately the first year. Here is a link to some Blazer specs with production number and differences in years.
http://coloradok5.com/specs.shtml.
Check out
http://www.collectorcartraderonline.com/ and do a search for Blazer and Jimmy's. Everyone pretty much covered the problem areas but also check the frame around the steering box. Second gen's ('73 & up) are famous for cracking there but I've heard of it happening to first gen's too ('69-'72).
Here is an image of the Rocker Boxes from Blazerworld.net. These boxes were rusted and cut.
