The one difference is a narrow strip of stamped sheet metal attached to the top edge. Most used Blazer tailgates, whatever their lower condition, still have this strip.
This metal strip is necessary for a good seal when the upper lift gate is lowered with its rubber strip. The tailgate strip is held in place with seven sheet metal screws.
On at least the 1971-1972 Blazer, the strip is also held in place by a long strip of double sided tape (like used to hold the decorative side trim in place). By removing the screws, cleaning rust and old paint, it can be attached to the new tailgate. Presto! You have a new Blazer tailgate.