Looks like a jimmy to me, jimmy grill, hood and the rest of the body seems to match. It is missing the trim pieces on the front of the fenders which make it look weird. In the pic also looks like the rockers might be missing or something, just darker in color. I would check condition of the rocker boxes which might explain the look like its raised in the middle at the doors and sagging on the front and rear end.
69 was first year of blazer and 70 was first year of jimmy. Grill looks like 70 to me. I have been told that GMC front fenders have the lip for the trim and chevy doesn't mine has chevy front fenders, doors, hood, tailgate and other various parts.
Looks like its been sitting without a sunroof for a while so check the rust really good.
I am also partial to first gens so if you are ready for some metal fab then go for it.
Check to see if it has power steering and power brakes, not just mechanical brakes. probably has power drums.