The original K5 was a short wheelbase truck. It was available in 1969 model year as four-wheel drive only; for 1970, a two-wheel drive model was offered. There were four choices for power plants: the 250 straight-6, the 292 straight-6, the 307 V8, and the 350 V8.
The Blazer was designed and marketed to compete with International Harvester Scout and the Ford Bronco. Both of these were originally aimed at the short Jeep CJ series, which were much smaller than other trucks.
The innovation of the Blazer was to simply offer a shortened pickup truck, which both increased interior space, and lowered the cost of production with a shared platform.