For what its worth, I had the garage door open and was wiping my black 69 ss car down today, with the tail of the car facing south, 1 pm in the afternoon. As I finished wiping the car down in the back, the tail panel color showed itself as being different to the body good enough for me to find the paint outline as it goes into the trunk at a 45 degree angel. The is no doubt in my mind, my car had its tail panel painted with a different black than the body. I rechecked data on this website and it appears to indicate black SS cars did not receive the black out tail treatment. Not so on mine.
I would say its 10-15% grayer in color than the deeper black of the body color. With the sun shining into the back of the garage appears to a good way to tell on my car. I hope to get a picture but it would be best judged by an expert, I did find the paint seam going into the trunk so I will try to get a picture of that. Anoter way to also check may be with a paint meter as far as the color difference verification.