I too, have been contemplating this idea. The hood on the C10 is a little different than the Blazer, but I think the this is doable.
After countless hours of looking at pics and comparing the grills together (I have both grill sets), the Blazer and the C10 bodies have a flat grill area that gets cut out to allow for mounting. Although the grill cut out areas are different, I would just cut out the required area for the 69 grill, just enough so the mounting can take place. That way, if you decide later on to revert back to 67 grill, just more cutting will be needed to fit it with the simulated radiator bulkhead. Both grills are the same size, but do have slightly different head light positions, and the back panels are not interchangeable. I don't have C10 body yet, or I would have tried this already. I do have an awesome idea planned for the C10 though. ;-)
Here are a couple of shots of the grills...I cutout the fake grill inserts so I could add something a little more realistic. Hope this helps to get you going.